Story related.
A strategy is hidden under the “SPOILER”
*** Spoiler - click to reveal ***This is the second to last boss. The skull headed traitor.
He is quite annoying but his attacks go in a pattern. First you can try to hit him with the sword. Block with the shield and hit him again. Try to time your block right so you block his strike and he will recoil and you can hit him multiple times. If not , no problem but only hit him once because if you try to hit him twice he will hit you again and it will cost you 1 block of health. You can do that a couple of times. After that he will teleport somewhere high and shoot bolts of energy. This is the time for you to start running around , hit some pots to collect hearts and ammo. Do not stand still , the energy bolts also take away quite some health. When he jumps down you will see a purple circle around him. Hitting him with your sword will cost you health so arm your bombs and normal bullets. Take out a bomb , put it on the ground , run away a little and when Mesmeroth is close to the bomb shoot his face with a normal bullet. This will stun him , the bomb explodes and the purple circle is gone. From there it’s just rinse and repeat. The pattern starts all over again. It is not that hard if you know what to do.
There might be other ways to attack him but this worked best for me. Just don’t forget to run around and break all the pots when he is up on a ledge shooting at you , you probably need all the goodies that you discover.