When the game says minions I think it means lane minions which are the ones that constantly come down in a line from the enemy base. Not to be confused with jungle creeps or the things other Gods can summon from their abilities.
This should be that hard to get, just keep spamming your 2 ability for the entire match and sooner or later you'll get 25 minions kills.
Modes like Arena, Clash and Joust are probably the best to play for this as minions are always pretty close even during team fights.