This is the boss that you encounter on level 3-5.
The battle consists of eight parts and takes place in a circular arena:
1) Rotating outer section that shoots bullets. Shoot it until it is destroyed.
2) Four nodes in the arena that shoot projectiles. Destroy all four.
3) Cone shaped object pops up near the middle of the area and shoots projectiles.
4) Similar to (1), but this time there are two rotating sections on opposite sides.
5) Similar to (2), but this time there are eight nodes.
6) Similar to (3), but this time the conical target moves more and shoots more projectiles.
7) Venom pops his head up and shoots webbing at you that slows you down. Spiders also attack from the perimeter. Shoot Venom's head.
8) Venom finally comes onto the arena surface and chases you (along with the spiders from part 7) around the area. Shoot his head again until he is defeated.
Here is my S+ run with Cactus: