Early game · first 8 minutes
Avoid standing in line with Sinclair and his Spectral Assistant when Vexing Bolt is ready. Move a bishop through a position he must use to shoot, then check the assistant before aiming a second attack at him.
Solomon vs Sinclair
What's giving you trouble?
Full matchup plan
Mid game · Walkers
Leave Rabbit Hex's marked area and use protection before the transformation prevents a new cast. Cover Sinclair's likely swap destination with another piece when practical, without spending everything on the assistant's old position.
Late game · Patron
Stay near cover and watch for Sinclair to use the ability he copied. Move the queen toward him after you see what he casts, and keep another piece near his assistant's position so swapping does not give him an easy escape.