Andaron Saga Trainee Promotion/Training Guide + stat gains/caps, by Terraformus
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Tier 1 (T1) base classes like Mercenary have promotion stat gains from the trainee class listed underneath them. The stat gains/caps of Tier 2 (T2) promoted classes, as they pertain to the trainees, are alphabetically listed and grouped for each trainee at the end. All trainee and T1 classes have a 60 HP cap and 20 for other stats. T2 max stat caps are in [square brackets] next to the promo gains.

Units do not keep class CON or Movement gains from T1 going into T2, e.g. Christina will have the same CON/MOVE as a Paladin promoting from Lance Armour as from Soldier/Cavalier. They will keep obtained weapon proficiencies, e.g. Elmer can use Dark magic as a Sage if he promoted from Shaman, though their weapon rank may decrease.

LUCK is not listed at all as it's always 0 with promotion gains, and the cap is always 40 in T2 classes. T2 will have base CON listed instead of growth - add/subtract the unit CON modifier to determine CON. Nat/Chrissy T2 classes are noted as (Female). Listed MOVE is base Movement, not gained amount. My recommendations at the end, plus some tips for initial training.

In older versions of AS you could use a Master Seal as a Level 10 trainee and promote mid-map rather than automatically next chapter, but this is no longer possible in the most recent official version and my bugfix. You should be able to in most fan mods as they use an older ROM as a base.

NATALIE
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Joins in Ch. 4 by visiting the house across from the Vendor (unique dialogue with each unit). Can be sent away in Ch. 4x to later return in Ch. 23 as a Hero with high stats. Starts as Level 1 Villager (Swords). No CON modifier.

Mercenary > Hero/Ranger/Swordmaster
2 HP    1 SPD
1 STR   2 DEF
0 MAG   0 RES
1 SKL   3 CON   5 MOVE

Thief [Skills: Locktouch + Steal] > Assassin/Rogue/Swordmaster
1 HP    2 SPD
0 STR   0 DEF
0 MAG   0 RES
1 SKL   1 CON   6 MOVE

Myrmidon > Assassin/Swordmaster/Hero
2 HP    2 SPD
1 STR   0 DEF
0 MAG   0 RES
2 SKL   0 CON   5 MOVE

CHRISTINA
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Auto-joins in Ch. 9, if Samuel wasn't defeated in any earlier chapter. Starts as Level 5 Recruit (Lances). -2 CON modifier.

Soldier > Hero/Halberdier/Paladin
3 HP    0 SPD
1 STR   2 DEF
0 MAG   1 RES
1 SKL   1 CON   5 MOVE

Cavalier [Skill: Canto] > General/Hero/Paladin
2 HP    1 SPD
1 STR   1 DEF
0 MAG   1 RES
0 SKL   3 CON   7 MOVE

Lance Armour > General/Hero/Paladin
2 HP    1 SPD
1 STR   2 DEF
0 MAG   0 RES
1 SKL   4 CON   4 MOVE

ELMER
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Joins in Ch. 8 by visiting the Inn then talking to him with Baudimon, Jakka or Estelle. Starts as Level 3 Pupil (no weapon ranks, can only use Magic Missile). +3 CON modifier.

Mage > Sage/Mage Knight/Druid
2 HP    1 SPD
0 STR   1 DEF
2 MAG   1 RES
2 SKL   1 CON   5 MOVE

Shaman > Sage/Mage Knight/Druid
2 HP    0 SPD
0 STR   1 DEF
2 MAG   2 RES
2 SKL   2 CON   5 MOVE

Monk > Bishop/War Monk/Sage
1 HP    2 SPD
1 STR   0 DEF
1 MAG   0 RES
2 SKL   1 CON   5 MOVE 

RALFNIR
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Joins in Ch. 16 by talking to him with Sigrid. Starts as Level 7 Journeyman (Axes). No CON modifier.

Fighter > Hero/Warrior/Berserker
2 HP    0 SPD
2 STR   0 DEF
0 MAG   1 RES
1 SKL   3 CON   5 MOVE                                       

Pirate > Hero/Warrior/Berserker
2 HP    1 SPD
2 STR   1 DEF
0 MAG   0 RES
0 SKL   2 CON   5 MOVE

Viking > Hero/Warrior/Berserker
2 HP    0 SPD
1 STR   1 DEF
0 MAG   1 RES
1 SKL   5 CON   5 MOVE 

Ralfnir can also become the Jarl class in Chapter 18, if he's your candidate for either battle, promoting from his current class into Level 1 Jarl. Still good to promote to normal T2 class beforehand for the stat boosts.

T2 Classes
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Assassin (Female, Natalie-only. Other female Assassins have 30 SKL/SPD caps) [Skill: Deadly Strikes (Gains 1.5% crit per point of skill, instead of 0.5%.) + Locktouch]
- Keeps Steal skill if promoted from Thief, doesn't obtain if promoted from Myrmidon
3[60] HP    3[32] SPD
4[28] STR   0[22] DEF
5[24] MAG   0[24] RES
5[32] SKL   6[20] CON   7 MOVE

Hero (Female)
7[65] HP    1[28] SPD
1[27] STR   3[25] DEF
0[23] MAG   2[26] RES
1[29] SKL  10[25] CON   6 MOVE

Ranger (Female) [Skill: Canto+]
2[60] HP    1[30] SPD
2[26] STR   3[22] DEF
0[27] MAG   3[25] RES
2[28] SKL   8[25] CON   8 MOVE

Rogue (Female)
4[60] HP    1[30] SPD
2[26] STR   3[25] DEF
1[20] MAG   3[25] RES
1[30] SKL   8[20] CON   7 MOVE

Swordmaster (Female) [Skill: Crit Boost]
3[60] HP    3[30] SPD
4[26] STR   0[22] DEF
0[20] MAG   0[25] RES
3[29] SKL   7[20] CON   6 MOVE
------------------------------------------
General (Female)
7[75] HP    2[27] SPD
2[29] STR   2[29] DEF
3[28] MAG   3[26] RES
3[29] SKL  11[25] CON   6 MOVE

Halberdier (Female)
5[70] HP    3[26] SPD
3[29] STR   3[30] DEF
0[20] MAG   1[25] RES
2[27] SKL  13[25] CON   6 MOVE

Paladin (Female - Anima replaces Bows) [Skill: Canto]
4[70] HP    2[28] SPD
2[27] STR   2[27] DEF
2[28] MAG   2[27] RES
3[30] SKL  11[25] CON   8 MOVE
-----------------------------------------
Bishop
3[70] HP    0[26] SPD
2[20] STR   3[21] DEF
2[29] MAG   2[30] RES
1[30] SKL   7[20] CON   6 MOVE

Druid
5[60] HP    3[26] SPD
0[29] STR   2[21] DEF
2[30] MAG   2[28] RES
0[26] SKL   8[20] CON   6 MOVE

Mage Knight [Skill: Canto]
4[65] HP    0[28] SPD
2[28] STR   2[25] DEF
2[30] MAG   2[26] RES
0[28] SKL   8[25] CON   8 MOVE

Sage
4[60] HP    1[29] SPD
5[25] STR   3[23] DEF
3[30] MAG   3[30] RES
1[30] SKL   7[20] CON   6 MOVE

War Monk
3[70] HP    0[26] SPD
2[28] STR   3[28] DEF
0[27] MAG   2[28] RES
1[28] SKL  10[20] CON   6 MOVE
----------------------------------------
Berserker [Skill: Crit Boost]
5[70] HP    1[27] SPD
2[30] STR   3[25] DEF
0[21] MAG   2[22] RES
3[29] SKL  13[25] CON   6 MOVE

Jarl [Skill: Crushing Blow (Skl % chance
to deal 50% extra damage) + gets the Jarl skill]
4[80] HP    3[30] SPD
2[35] STR   1[30] DEF
0[20] MAG   1[26] RES
2[30] SKL  15[30] CON   6 MOVE

Hero
4[70] HP    2[27] SPD
3[28] STR   2[27] DEF
0[23] MAG   2[24] RES
2[30] SKL  11[25] CON   6 MOVE

Warrior
4[75] HP    3[26] SPD
2[32] STR   1[26] DEF
0[20] MAG   1[22] RES
2[28] SKL  13[25] CON   6 MOVE
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Which classes to pick though, and how to train them to the first promotion? For the former, it's fairly cut and dry in terms of effiency and usefulness, but any class can work. I suggest always promoting at Level 20 since you'll easily cap by late game and will need the stats.

NATALIE: Mercenary is the go-to, or Thief if planning on Assassin or you're lacking in stealers. Ranger and Assassin are the optimal picks, Hero and Swordmaster are okay, Rogue sucks. Her Assassin and Swordmaster classes are a bit better than the normal female ones for SKL/SPD caps. Obviously if you don't want to train her just send her away in 4x (don't skip dialogue) and you'll get a good Hero for free, much later.

Training for T1: you can chokepoint Wilhelm in Ch. 4 and let him break his weapons before grinding him down, though she may kill him before getting to Level 10, so I recommend getting a few levels before then. In Ch. 6 you can put her on the nearby northern pillar and grind on the strigoi/gargoyle, and otherwise fight some driders. You can also kill eggs but it's inefficient since the flat 50EXP is better for higher level/promoted units. Ch. 7 has two initial stationary archers, and some weaker cavalry, but if she's still not ready to promote by Ch. 8 preps I wouldn't use her.  

CHRISTINA: Cavalier > Paladin, simple as. Real chads make Christina and Amelia Generals though! Note that Halberdier lacks Crit Boost. Hero does have fun lance animations.

Training for T1: ideally get her to Level 10 on her join chapter; the stationary archers to the south are good targets and she can contribute with fighting cavalry. Ch. 10 and 11 are faster paced chapters with lots of axe enemies; she can get some EXP from initial brigands in 10 and nearby soldiers in 11, but it's far better to promote after Ch. 9.  

ELMER: either Shaman > Mage Knight/Sage or Monk > War Monk, but it depends on team composition. Generally I'd recommend going Shaman due to how good Dark magic is. Note that visiting the Ferdinand house in Chapters 10 and 20 (Ch. 9 house always gives him a Fire tome) will reward a weapon for him depending on his class, with T2 classes getting stronger ones - of particular interest is Shaman > Mage Knight offering a Light Brand AND Nosferatu in Ch. 10.

Training for T1: on Normal and Hard difficulty you can easily chokepoint the boss in Ch. 8 in-between the houses to the East and chip him down, but on Lunatic it's tricky to level him there due to death squads. Ch. 9 has plenty of opportunities like finishing off cavalry, killing armour knight reinforcements from the north bridge, stationary archers etc. I wouldn't bring him as a Pupil and try to level in any later chapters.

RALFNIR: Jarl is by far his best class and is a separate scripted promotion, but as mentioned earlier you'll want to promote him into a T2 before winning the combat in Ch. 18 for the stat boost. I suggest Pirate > Hero as these offer the most balanced offensive stats that compliment Jarl caps. If not going Jarl (or, you're wanting to prioritise his weaker defensive stats), Viking is nice for the CON boost and to shore up his weaknesses before the levelling grind, then either Berserker or Warrior depending on your team.

Training for T1: you absolutely should get him to Level 10 when he joins, he's close to it anyway. If you don't care about Jarl then just drag him along in later chapters and fight weaker enemies, but generally if you want to use Ralfnir you're going for Jarl, and for that he should get to at least T1 class Level 10 in Ch. 17 so he can promote for Ch. 18. You'll likely want to put him in the southwest since most of the enemies there are melee and/or use lances, aside from the driders, equipped with a Holy Axe. Some statboosters and Holy Symbol will help too. Taking the deal will let you take your time training him and potentially let you reach level 20 then perhaps higher in T2, but locks you out of the normal ending and bonus chapters Ch. 32 and 32x.