Early game · first 8 minutes
Keep Arctic Beam on Rat King while he fires to reduce his fire rate. Fight near cover so you can step away from Scrap Grenade without losing track of him.
Kelvin vs Rat King
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Mid game · Walkers
Use Ice Path to reach higher ground when Rat King advances with the banner. Its slow immunity prevents Arctic Beam from slowing his movement.
Late game · Patron
Use Frozen Shelter on an injured teammate before Rat King reaches them. Keep him outside the dome while they recover, then use Ice Path to help them move away.