Dark mage

the dark mage appears almost throughout the entire game and is second to the ghost mage as the second most powerful mage it is known to taunt you throughout the ends of stages in the game for three stages: Black marsh, icy plains, and volcano. at the end of black marsh and in black marsh it was at its most powerful form

black marsh
for the first time ever the dark mage appears on the lower deck of the ship on the final level of black marsh along with a squadron of green mages, the goal of this level is to keep the bombard and one of the catapults. without it the level can't be won without true aim mystery (only on mobile) and the correct level of upgrades and the right luck and smarts. to beat this level make the metal balls roll down to the bottom deck to weaken or kill the dark mage, if weakened throw the catapult or bombard bomb onto the dark mage. then shoot the bombard below the ship to kill the Green mage on the bottom, and kill the rest of the green mages on the ship if you didn't already.

icy plains
the dark mage appears in icy plains again and is in the throne on the bottom of the ice fortress and is accompanied by two sky mage's. So after black marsh his assassination is easier, go through the wall holding up the ice fortress, hit him, and get rid of the rest of the mages. easy right? once that's done get the sky mages on the sides of the throne and kill the frost mages on the top.

in volcano
In volcano the dark mage appears in another fortress incased in the walls making it close to as hard as black marsh as volcano stone is harder to break than limestone. he has volcano and fire mages protecting him, I have no tips on how to help you with it.

At the end of the game on citadel.
The dark mage appears one final time at the last level and appears beside all the rest of the mages that appear throughout the entire game. he is minor this time as the ghost mage is in the hanger making it so you have to shoot the brick on the top holding a metal ball and then it releases all of the mages killing some and leaving some lucky to survive. once you defeat them all you get to the end of the game.

Other
the dark mage does not just appear at the end of the stages, he can also appear in the levels in the stages and be there to stop you but this happens rarely besides in volcano and citadel as he appears more in both stages.

and he did not make it as the final mage in the last cutscene