In any case, the trap can be used to stop the enemies' advance, so you lose nothing this way. Use it for that and be happy
Oh, wait. in addition to stopping them, you want a medal for that. Well, in my opinion, that is a bit selfish. If you want the medal, do it the proper way. There are 300 battles in the game. Successfully trap 1 unit each 3 battles (and that doesn't cost a lot of nerves) and you will get the medal.
And with all due respect, nobody forces you to get that medal. It doesn't do anything to improve the score. In fact, as a warrior and paladin, where you need to take the fight to the enemy, those traps on the battlefield will simply hinder your tactical options. As such, I fail to see what is the problem and why are you unhappy.