Defeating the Demon Lord's a Cinch (If You've Got a Ringer) Light Novel, Vol. 1

What's a healer to do when his party's swordswoman is useless because she just changed her fighting style, the mage can just barely use one type of magic, and the all important hero does nothing but glomp onto any beauty in sight??? Support them from the shadows and do all the heavy lifting, obviously. The demon lord won't stand a chance! Probably.