For necromancy, you can use it just by interacting with skeletons or some corpses found around the map.
You can't kill the head of the wizard. You can have him join your team if you talk to him during battle, after acquiring the eclipse talisman. Not an easy task in and of itself because you get the eclipse talisman from talking to a yellow mage about gro-goroth. Which means taking time during the already dangerous yellow mage fights in order to have a conversation.
As for the mauler, it's a dangerous enemy. You can actually beat him early on if you pick up necromancy and walk around with an army of well armed skeletons, but it takes a lot of luck to do it that way since you can't control the skeletons in battle. A much better option is to get attack spells on multiple party members and nuke his head (you can do it with just one party member knowing good spells, but there's a real danger of him pecking one of your team members head off or mauling you to death, if you've got expendable people on your team you might consider taking that risk). It's head alone has got several hundred health. About 500 if I had to guess, but you can do that much with two casts of black orb or greater hurting, which will also almost always hit.
Have you found a way into Ma'havre yet? The city of the gods? That's required for a lot of the endings.
Edit: For those having trouble getting spells, there are a few ways. First is to get a soul stone and use it with an enemy you've beaten or an ally whose died. If you've gotten some affection from gro-goroth and you take the soul to one of those hex table things you can learn his damaging spells. You can find one on the first floor behind the cursed door in the courtyard. Another way is to get a random "scroll of black orb" or "scroll of pyromancy" or other ability scroll item from bookcases. These are the only way I know of to get spells for people other than your main character, so I'd save it for other party members that you plan to keep, personally. The third method is to get an empty scroll and a quill. If you use the first verse as "O LORD" the second verse as "TEACH" and the third verse as the name of the spell or skill you want (for example "HURTING"), All-mer, one of the gods, will teach you pretty much any spell or ability. You can learn to do this in game if you pay attention to some of the book pedestals you walk past. Some of them will have examples of how to use the scrolls, like the exact one I used here.
You can also use an empty scroll with "O LORD" and "GIVE" and the name of certain items. You can use this if you've messed up and locked yourself out of getting some item you need to proceed with specific endings like the eclipse talisman. Specifically the third lines of that is "ECLIPSE".
