With the unacceptably poor perameters we're given to judge the event, I'll imagine this is taking place inside of some kind of very large barren arena with no other resources or places to hide.
400 men killing 700k cats would be virtually impossible, since cats are slippery little bastards even for those not in heavy armor, and cats don't willingly engage in straight up combat very often, so they'd just run away.
Then the war of attrition sets in.
Google says cats can survive roughly 3-4 days without water while humans can survive for a week to 10 days without water. That would suggest the humans would win out just by waiting as the cats died of dehydration. Plus legionaries were equipped with waterskins and 14 days worth of food according to wikipedia, along with javelins they could use to better hunt the cats if necessary and cooking equipment, giving them the obvious leg up.
The problem is that as the 700k cats began to die off the area would begin to fill up with bodies, which would complicate the matter since the cats could, uh...reclaim some moisture and nutrients from their fallen comrades. Plus disease resistance over time would become a question. However cats are very resource intensive creatures, and I think even factoring in the immense biomass of over 8 million pounds of cat, I still think the cats would probably wind up dying of illness and dehydration before the humans would.
So 400 Roman legionaries.