So... in the rulebook it says that during combat with another player, each player uses a unit, or a tactic at a time.

"Mind control" is a tactic card, and "Battle Cry" is also a tactic card, but with minor change in description which says: "Play during combat".

my question is: may I play a tactic card that doesn't have the line "Play during combat" (like battle cry and summon lightning) during battle? the rules never mentioned anything against it. something is awfully wrong playing mind control during a battle, waaay over powered.

Thanks in advance!

Action abilities only trigger during the action phase of a player's turn. Only during this time are you allowed to play cards for their actlion abilities. It's in the rules on page 7 and 14.