Painkiller

Saxat från gamefaqs

"For this boss you can't beat him by shooting him directly. Instead you should
fire at the planks of wood in the middle of the star in the ceiling. Walk
across the ledge over the middle of the pit of fire, then wait until the boss
is very close to you before you take a running jump to the ladder on your left.
Climb up the ladder as the boss falls in, otherwise repeat the process if you
jumped to early. Once the boss is dead you can walk across the middle ledge
once more and jump to the level exit."