Key to this achievement is card named Demon Lord, t gives 2 strngth at the start of your turn. It's good to have other cards giving strength as well, but that is not necessary. Best opponent for this is at level 3 - those 3 black puddings that regenerate. Just make it so 2 of them are dead while 3rd one is up. Next turn kill 3rd one while 2 are up,control the situation (with growing strength ths shouldn't be a problem) and eventually you will reach 50 strength.