I don't find what the player is doing as metagaming at all. As a DM if I find a player is using the same combo of actions over and over again, I place intelligent oponents that try to counter them ala fire resistant creatures against a pyro wizard.
I think it perfectly reasonable for an intelligent adventurer to realize "hmm I seem to get attacked by a lot monsters just waiting for me to cast a spell, I better come up with something to counter that."
I would reccomend changing your tactics a bit. All DMs can get a bit stale in how we use our creatures, if a simple and creative tactic like this is freaking you out, dont nerf the player, change your tactics.
I think it perfectly reasonable for an intelligent adventurer to realize "hmm I seem to get attacked by a lot monsters just waiting for me to cast a spell, I better come up with something to counter that."
I would reccomend changing your tactics a bit. All DMs can get a bit stale in how we use our creatures, if a simple and creative tactic like this is freaking you out, dont nerf the player, change your tactics.