Windows 10.

Can't kill the final boss.
(spoiler) I do:
I shoot his both legs, then shoot in his hole and head about over9999 times and he still staying alive.

If you had his lower legs blown/shoot off and are in the "He tries to suck you in, expose his weak spot at the head and sc reams to you"-cycle, he should die after you hit his weak spot for 5-10 phases otherwise it should be a bug.
The game isn't supported anymore (studio closed 2013), you could try to send you save to EA and pray :) or reload a older save and replay the fight.

You cannot use guns to kill him.

You need to use magic based abilities to kill him like throwing the summoned skulls.

You need to shoot this "head" with skull storm spell until fucker die. For me it was more than 5-10 times for sure, more like 20.

I think this whole battle is pretty bad design. I needed to check youtube because I would probably never guessed it's just about brute force. I was sure there must be some special way for killing it, like putting dynamite in this vagina (especially because there is dynamite on the island. What for?!) which seemed quite logical and I tried to do it for an hour or so until I gave up.