Early game · first 8 minutes
Hit Rat King with Alchemical Flask before trading gunfire to reduce his weapon damage. Use Binding Word after he spends his dashes so he has less chance to escape.
Warden vs Rat King
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Mid game · Walkers
Use Binding Word when Rat King follows an injured teammate. Move beside cover with Willpower while you shoot him; immobilizing him does not stop his gunfire.
Late game · Patron
Start Last Stand behind cover after Rat King slams his banner down. Move toward the enemies around the banner once the pulses begin, using Willpower to stay close enough to damage them.