Also read PHB p. 183, Charm Person. It there states that the target receives a +5 bonus on his saving throw if it is currently being threatened of attacked by you or your allies. So if you cast it in a combat, you'll have a Spell with a DC of 6 + your relevant ability mod (10 + 1 for 1st-level spell - 5). That will be around 10, or maybe 16 (for a really smart guy with greater spell focus). Not so much to beat if you've a couple of class levels, and even if you're only 1st level you have a good chance of beating that! So on higher levels it's almost useless, and on lower levels you find better combat spells. (Remember, the guy won't attack his friends, he won't do anything that's against his alighment....). If you want him out of the fight, use sleep. It will put him (and probably some comrades of his) out of the whole thing, at least for a while.