Early game · first 8 minutes
Place Spectral Assistant away from you so Scrap Grenade cannot hit you both. If Rat King throws it at you, swap to your assistant and fire Vexing Bolt from there.
Sinclair vs Rat King
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Mid game · Walkers
Use Rabbit Hex as Rat King gets close, then attack with your teammates before he can activate Royal Pestments. Hexing him after the barrier appears will not remove it.
Late game · Patron
Place Spectral Assistant beside cover before approaching the enemies around the banner. Use Rabbit Hex on the group when your teammates start shooting, then swap to your assistant if Rat King turns toward you.