Early game · first 8 minutes
Charge Power Slash behind cover, then step out to hit Rat King. If he gets close, use Crimson Slash to heal and reduce his fire rate.
Yamato vs Rat King
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Mid game · Walkers
Wait for Royal Pestments to end before using Flying Slash to reach Rat King. Follow with Crimson Slash and Power Slash, keeping a ground or air dash for rats he attaches while you fight.
Late game · Patron
Use Shadow Transformation as Rat King starts to slam his banner down near you. Its initial invulnerability can prevent the damage; follow with Crimson Slash and Power Slash after your abilities refresh.