In my opinion
Short answer: just use strength minions.
Swordsman has most power. Fast attacks and nice dmg it can kill str monster one on one. So as a mid level player you might want to mass swordsman. Problem is that at higher level, Ruul players will have enough damage to kill them easily, and Ameryl will again kill them with “punish arrogance” spell (this does dmg based on minion atk). And for Mordrake it does not really matter.
Archers: They are weakest on their own, but if you manage to get some numbers you are in for a win. Again at higher level Ruul can one shot them and Ameryl can one shot them, but very good vs Mordrake (I might even recommend always making archers vs Mordrake). So if you are low lvl go archers vs Mordrake and Ameryl but never against Ruul.
And strength monsters are decent, but altogether probably best because nothing can one shot them and can tank quite a bit. This will cause minions running back to kill them after charge to a boss > Longer time they survive gives more time for next monster to charge etc. and at the end that is how you win a game by constantly charging to a boss. Well either that or by counter attacking with large group of minninons, for which str monsters are again best because they are less impact by aoe spells.
One more thing to mention, placing a mix of those units just does not work for me. Have a feeling like in this scenario non of the minions contributes.
Of course feel free to experiment and please share if you have a killer tactic