I've noticed that my setup (elf priest with 2 human warriors) is weak to spark wizards, so I'm trying to find some good armor with Grounding Plates. Any suggestions? If not, anything else I can try?
I like Stormward Pauldrons for non-dwarves. The encumbrance on Heavy Armor can be annoying but it prevents a lot of damage and you can usually discard if you need the movement. Blocks aren't very good against sparks since they have hard to block so good armors like Reliable Mail are your best bet. You can also try Impenetrable Nimbus but that only buys you one turn. Spark mages can't hit you in cover, with the exception of linear spells if you're near the edge, so get behind something if you need time.
I don´t know which helmet you use but you could combine Crusty Helm with Perilous Ringmail. Stacking armor is great against fire wizards and it also helps a lot against spark wizards. Both have relative harmless traits on them and with a priest on board you can handle the extra damage from the curses. With those two tokenless choices you might be able to adjust your build further.
Aside from Reliable Mail, I ignore armor and go for team moves. Stay behind cover until you can go for the kill. Also what Questor said.
