Magicka: Dungeons and Gargoyles Achievement: The others were dwarfed by you

I got the achievements for beating the mage last and the champion last on the same fight.  I think it doesn't take the champion's fairy resurrect into account.  Anyway, I was having some trouble leaving the assassin for last, as he dies so easily, but I finally got it.  Being humbly awesome, I will share my strategy.

The champion has a lot of damage reduction and a fairy not unlike your own, meaning you'll have to kill him twice.  I usually concentrate on him first, because he has to be killed twice.  I'll have none of that shenanigans here.  I spend most of the fight in his face dealing damage.

The mage teleports both at random and when you get close to her, with a cooldown that can be used to get hits in, but generally it's best to keep your distance from her because that cooldown doesn't last forever and you'll soon be at a distance from her anyway.

The assassin hops around, can cast spells that rain elements down on the stage area, and will also spam poison darts.  He actually ends up poisoning the mage most of the time, so if her health bar turns green you can ignore her for a while.

On your way to this boss fight, make sure to grab all three parts of the letter so you can get Teleport.  Teleport is really handy in this fight.

Spells you'll need:

• EDDDD - The full rock shield.  You'll be in the champion's face for most of the fight, and when you aren't you should be teleporting towards him to get in his face.  Thus, the movement speed penalty from this shield won't be affecting you much.  After every couple of hits from the champion, refresh it.
• DQRQRQRQR - The spiky ice rock.  Fully charged, this is the most damaging single-hit spell in the game.  Even when not fully charged, its damage is respectable.  You will be shooting the champion with these constantly.  Even when he dies the first time, start charging another one instantly.  If you get the chance to pop off a fully charged one, do so, but any level of charge will do.
• EQ - Water shield.  The majority of the damage from the mage is water-based.  You'll have this up while you're healing from poison damage, and then switch back to the full rock shield afterwards.
• W - Healing.  Eventually the poison damage from the thief will get to be too much and you'll need to heal yourself.  Be careful when you do this, it's best to have some distance between you and the champion when you decide to heal.  If he starts coming your way, even a quick ED will save you.

The strategy is as such: Before triggering the fight, toss on the full rock shield, and haste to the cutscene trigger.  Haste across the room and start charging a spiky ice rock.  Fire it at the champion as he closes in.  Once you get into a rhythm of ice rock, refresh shield, repeat, watch your health.  If you get poisoned at all, self-heal to get rid of it.  Even with a rock shield up, you can still get rid of poison.  If the champion starts chugging health potions, hit him with anything, immediately.  Once he dies, quickly heal up, refresh your shield, and begin charging another spiky ice rock.  Once he dies the second time, you're good to go.  Charge up a spiky ice rock to full and carefully aim it at the mage.  If for some reason she's still alive, repeat until she's dead.  Follow up by finishing off the assassin however you see fit.

I hope this post is useful to someone, because I certainly had a hard time until I remembered the spiky ice rock spell.  The key to this fight is being able to knock large chunks out of their health quickly.  My previous strategy involved a water-rock shield and ARSE mines, and while it was working, it dragged the fight out much longer than it needed to be, and usually one of the three would glitch and disappear.  Sometimes it was me who glitched and disappeared, and I had to teleport to get back into the fight.  Since the fight is so glitchy, bringing a swift end to it is the best solution.