Ygg

General
Ygg is Magicka's first real boss, not counting Behold the Watcher. He bursts in the large doors to the outpost north of the Castle Aldrheim. He grants the achievement No more trolling.

Defeating Ygg
From the Chapter 1, Area 5 Ygg Guide:

Ygg, the aforementioned Forest Troll comes charging into the camp. It is large and carries a large wooden tree trunk. It moves relatively slowly, especially if it is slowed with the Cold element. It regenerates hitpoints all the time but this can be stopped by setting it on fire. It does three types of attacks.
 * 1) Perform a swing of the wooden tree trunk, knocking back everyone standing in front of it for a large distance and doing moderate damage.
 * 2) Grab someone (preferably not you) in his hands and slowly eat it until it dies. If this happens, press spacebar repeatedly to shake free or use an area stun spell such as +Shift+Right click.
 * 3) Kick someone in front of him for a small amount of damage, knocking it backwards for a short distance.

There are 3 main methods of killing Ygg:
 * 1) Kill it with a huge damage spell. Use the recommended  beam.
 * 2) Heal the guards attacking it with a healing beam  and they will eventually kill it.
 * 3) Charge a Nightmare Hailstone ( force) and release it when there will appear a free (of soldiers) path for it. Will kill Ygg in one shot.

There are 2 possible outcomes to the battle (that doesn't involve you dying):
 * 1) Ygg dies and Captain Bjorn (the soldier in the red cape) dies. A soldier runs in from the northeast, moans about his death and tells you to go on through it on Havindr. Pick up the Captain's Axe dropped by the dead Captain Bjorn if you wish.
 * 2) Ygg dies and Captain Bjorn lives. He congratulates you on your prowess and because you're so good, you are to continue on towards Havindr without his help (he's supposed to accompany your journey). He gives you his Captain's Axe as a reward.

Killing Ygg will complete the "No more trolling" achievement.