Well, here is a suggestion for you. This boss will require the players to spread out and to move a lot.
Magnataur Lord
Phase 1:
In Phase one, the Magnataur does not attack with his melee attack, but simply stands in the middle of the room and casts.
Blizzard
Channels for 3 seconds a blizzard on a random player, dealing ice damage. (Same as ordinary blizzard)
Magic Spear
Throws an ice spear on a player and all players within 512 AoE, dealing ice damage and stunning them, but after the stun duration, the spear is given to them as Item. They can throw it back for damage and stun (stun does not stack). When the carrier dies, the Spear is lost.
Frost Nova
Deals a huge amount of ice damage in an AoE around the Magnataur. All units hit get an 60 seconds debuff that greatly reduces ice resistance. Automatically casted after a stun from a Spear.
Immobile (Passive)
The boss cannot be taunted, pulled and kited. He stands always at the same position.
Phase 2:
Phase 2 is entered at 75%, 50% and 25% Health. The Magnatuar immediately casts Frost Shield, summons three Tuskarr and looses his immobility, means he can be taunted and kited, but also begins to attack Players with his strong and splashing melee attack. Phase 2 lasts 60 seconds or until the Shield is broken.
Frost Shield
The Magnataur takes only minor damage from every attack and spell. The Shield can be broken by throwing a Spear at him, thus making him return to Phase one.
Cleave (Passive)
25% of the dealt damage is splashed.
Tuskarr Servant
Summoned by the Magnataur Lord at the beginning of Phase 2. The Tuskarr stand at the rim of the Battlefield and continue to exist even after returning to Phase 1. They have low life points.
Magic Spear
Same as Magic Spear of the Magnataur.
Immobile (Passive)
The tuskarr cannot be taunted, pulled and kited. He stands always at the same position.
To kill this boss, a lot of tactics is needed: He is only vulnerable during Phase one, but the Frost Nova prevents the melee attackers from attacking him because the damage dealt kills low armor-classes and the high-armored are left very vulnerable with low hp and damage amplification for all icy spells.
The Blizzard and the AoE of the Spear forces the players to stay away from each other and to move around a lot because the damage dealt by a Blizzard is enough to kill low-armor classes. To damage the boss, the players have to collect the Magic Spears thrown by him and his servants. But that can be difficult since the Spears stun and are lost when the Players dies, meaning that you have to prevent the boss from casting a Blizzard on a stunned hero.
The Tuskarr can provide you with more Spears, but also increase the amount of stunned players and damage dealt.