Early game · first 8 minutes
Apply Static Charge to Rat King, then send Lightning Ball through him as he is stunned. Keep a dash for rats while you aim.
Seven vs Rat King
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Mid game · Walkers
Use Power Surge after Royal Pestments ends. Shoot Rat King when he stands close to his teammates so the shock damage can hit them too.
Late game · Patron
Use Storm Cloud after Rat King slams his banner down. Start from high ground with a clear view of his team; enemies behind solid cover will not take damage.