sometimes you just can't spare that 1 mana. in a low mana battle, that's the difference from being able to kill the enemy backline and losing to a cube. also a cube starts with a lot less hp, so some strategies simply can't work with it. there's also the healed out ruleset preventing heals
the number one factor in rise of the commons is, monsters just don't have that much damage. get a peaceful giant, a healer and debuff the enemy's attack. you are likely going to win.
ie: bortus / zintar / contessa, peaceful giant, snipers, debuffers (silence, headwinds, demoralize)