Summoners are really good because you can summon them on top of different elements and do physical or magic damage. make sure to take advantage of that.
Also, if you haven't kissed the spider lady, do it with all your characters and choose the Transparent Chrysalis option. You get an idol that you can revive you instantly. You can recharge it by combining it with resurrection scrolls. This effectively gives everyone two lives or you can put multiple on one character.