So the zombie run is going well, and your tips are helping out! We got contamination and poison dart on everyone, so it turns out that we have enough chip healing to do some work if me and my friends all pitch in on healing. It requires us to stack up and stick together, though, which is a huge pain, and fire is absolutely as devastating as I feared. The fire slugs wasted us. Like, it wasn't even funny. Also, I have yet to find any poison grenades or arrowheads, though I've been checking all the chests and vendors. I have a feeling this will get nightmarishly hard once we start running into things with fire in the second half of the act, so I am still very skeptical at this point. I'm specifically thinking of decaying pyromancers, archers with fire arrows, and the mage in the last boss fight. At best, this is going to be hell. Just beat Kniles, by the way.
