Spider is pretty powerful already. Poison attack, unlimited Web spells (which can be set on fire as well,) and more hp than the others. I tried all of them and only like the spider. Try stealth, get your spider in position maybe to cast Web first, then attack with your others. Only get the spider close if you want to use the poison attack, otherwise continue casting Web and moving. Spider can jump behind the webbed enemies, if the rest of the group continues to hold their attention then the spider can stealth and move on the next turns until you need another Web or poison attack.
