Gardan

Gardan is only boss of Killing Grounds dungeon and Gardan's Trial solo Ace Dungeon.

Gardan exists in few forms: KG is part of "single-boss only" dungeons that devs started making after Exodor release based on player feedback (cf. CS, FA, DA, Catalepticon, KG and FL). Well, players hated these new dungeons too, where mechanics of bosses are slightly more complicated that before. TBH Killing Grounds is one of better ones of this set. Gardan does a lot of AoE, but it's cleverly hidden into boss attacks around him, not on some dumb telegraphed attacks.
 * Kaia's Soul Gear "season" in Killing Grounds group dungeon and Crimson Killing Grounds hard mode dungeon
 * as solo Ace Dungeon boss

Knockdown and big "Stomp" attack
At 85% boss will knock everyone down and start a big attack (3 possibilities, see glow color on boss axe and legs). You must stun Gardan before this happens, or it likely wipes out most of the party.

This mechanic does not happen in Ace Dungeon solo version.

Ice Debuff
Player who is most far from boss gets debuff.
 * "Those guys back there... are infuriating..."

Player who is closest to boss gets debuff.
 * "The guys nearby...are annoying..."

If you take 2 debuffs you instantly freeze and die. Repeats every 45 seconds, debuff lasts 90 seconds.

Snowballs
Giant chained on wall spews out ice balls. Targeted player is marked with secondary aggro. Place puddles away from boss since they apply a frost stack on you.

Mechanics repeats every 45 seconds(?).

Wave
From 80% on Gardan occasionally releases a wave. He first holds hand up.
 * If hand has red glow, wave is out from boss.
 * If hand has blue glow, wave is in from edges to the boss.

Unleash
This is similar like berserker's Apex Unleash skill. At 65%, 35% and 1% Gardan transforms, gets two axes and different attacks, pull all players into himself, arena is surrounded by wall.

Every hit you take applies a frost stack. You freeze and die on 10 stacks. Occasionally boss goes after random player, attacking them until his combo ends or player or boss dies. It's possible to drop aggro to avoid this (sorcerer, archer, priest). On worst case hide behind tank, who must hold block to protect player.

At 1% boss goes temporarily immune to damage, goes unleash and casts a shield that is about 10% of his total HP. Surprise! This unleash only ends when boss dies.