Just in case you don't have enough hydro characters, cryo can break the Fatui's electro shield, and electro can break the Crab's pyro shield. However, this only works on Menacing difficulty or lower, because I tried Fearless and the cryo just isn't enough to eat up the shield compared to the regen rate.

What I find a little weird is, the Crab's pyro gauge is not actually an aura. I tried it with Clorinde before, and she couldn't trigger Thundering Fury's 4 set bonus even though theoretically, she should be triggering overload. But no, her skill CD remains 8s.

Hope this is helpful.
Alternative to defeat these two in lower difficulty