You need to have Dwarf Turret as one of your abilities.

As a second ability pick something like Stomp for free npc help on the last boss and to clear the blocks on the boss level faster if you are teleporting near level 10.
If your are starting from level 1 then pick something that can help you easily clear levels while being a safe distance away like Dragon.
Another option would be to take Shield or Sanctuary which are helpful in both cases.

For passives it doesn't really matter what you choose to have but, it is easier if you dont pick abilities that start you at less then 100% health like Wealthy or Aggressive.
Phoenix is a great ability to take in case you get hit by an unlucky Sand Worms or Raise Dead.
Slayer is also very good if you need to clear levels to reach the boss.

The basic method is to have your Dwarf Turret army kill the boss from afar and destroy the blocks at the bottom of the level so if he does come close he will drop onto the spikes.

When the level begins Stomp or something similar to release the npcs, if you didn't choose an ability like that then use your keys to free the Wizard or Paladin.

Next drop down to the bottom of the level and place your 2 Dwarf Turrets on destroyable blocks for when they blow up.

When the boss spawns wait at the bottom for a few seconds since usually the boss will do an area attack when he spawns and jump to one of the platforms to the right or left if you aren't in view.

If the boss does jump on the left or right platforms out of view (and not immediately down to you), then jump up onto one of the blocks above so they will barely be in view (you can see them but, abilities like Dragon can't hit you). At this point the Dwarf Turrets will shoot him to death while he jumps up and down on the platform.

If you have any block destroying abilities then use them before the next round starts to try to make a path to the spikes at the bottom.

The first few times you kill him the Dwarf Turrets wont be fully used up so put another 2 down after stomping/keys at the beginning of the round and by the time the hard versions start you should have quite a turret army built up to easily kill him.

Always remember to use your turret ability twice before the next round starts.

After a couple stomps or turrets blowing up there should be a path to the spikes so you can "shove" or lure him down onto the spikes if he does come down to you.

Video provided by Damager_Psytrickx.

Watch out for Sand Worms, they can spawn very close to you and kill you fast if you dont move out of their way (or atleast don't take in their full hit), try to stand in a place you can move away from easily such as a single block.
Watch out for Raise Dead, you can have 3 or 4 of them spawn on top of you and kill you very fast, again try to stand in a place you can move away from easily.
Watch out for the Death Doll, be sure to stand in your Sanctuary or have your Shield up if you can.

Tips would be that you can kill him fast by yourself and save turret strength in the beginning rounds, if you took Shield then you can kill him easily with no chance of dying. Spear also helps to keep him at bay and get some safe hits in (depending on his skills).
When you finally do clear a path to the spikes you can "shove" him down by using a spear, sword n board, or knockback abilities to hit him down the hole.
If you got him down the hole and fear him coming back out (this only happens near the end when all destroyable blocks are gone) then you can stand at the edge of the block he is trying to jump to, this will knock him back onto the spikes (If he has a Dragon or something similar then do not block him, stay far away and try to lure him down another hole).