For two of those missions, use flower knights in your squad that increase speed and ignore spiderwebs. (Engage with the pest chasing you = guaranteed squad wipe)

For the last one, things are a little different. It's designed so you can actually beat the Upgraded Slug chasing you. I think you're supposed to let it get hit by a bunch of Chibi-Nazunas to whittle its HP down, but I don't think that's enough on its own (it still has the one-hit kill skill, but only targets 1 girl at a time instead of the entire squad, still takes a LOT of hits to beat.) What worked for me here was using a bunch of girls with evade, that allowed them to dodge the OHKO skill most of the time (some with counterattack too to get more hits in) and sure enough, it was GG!