Mash your shovel on the spot where subterror went under to dig him up. Then either slash him, or punch him with the expert's ring. The ring does as much damage as a spin attack, so you can kill him in one cycle if you're lucky.
Every time the hag appears, you can get 3 hits in with your seed shooter by bouncing the seeds off a wall. It's easiest if you kill the bugs so they don't get in the way. It dies in 6 hits.
The best way to get 3 hits in is to stand about 2 tiles away from a wall, do your first shot from there, and start moving towards the wall after the hag spawns. This keeps her visible for the largest about of time, while keeping the wall close enough for 3 seeds to reach her.
Note that you can only fire one seed at a time.