Early game · first 8 minutes
Use Splatter when Rat King stands beside the wave. If he throws Scrap Grenade at your lane partner, use Puddle Punch to move them away from it.
Viscous vs Rat King
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Mid game · Walkers
Use The Cube on a teammate Rat King is shooting after rats attach to them. While they recover, use Puddle Punch to push him away from them.
Late game · Patron
Use Goo Ball after Rat King slams his banner down. Hit the enemies your teammates are shooting so your allies can keep damaging them while they are stunned.