Tips to Use Tusk (Dota2)

• When being pursued by numerous heroes, you can cast Ice Shards toward them to hinder their movement, as the barrier forces them to break up and even come to a slight halt. It is ideal to cast Ice Shards when your pursuers are in a small choke-point, as the barrier will most likely block their way, or at least make it even more difficult for them to move forward.

• Ice Shards' missile cast is delayed; keep this in mind when you are targeting distant, fleeing heroes.

• When pushing, defending, or farming, Ice Shards is an effective area-of-effect skill you can use to damage multiple creeps in a straight path; unlike target heroes, Ice Shards doesn't create a barrier when it contacts creeps.

• Aid your team's initiator by pairing up with him/her and casting Snowball on the group of enemies; the movement makes both Tusk and his teammate invulnerable, and any target that is hit by it gets stunned for a brief second, giving the initiator some time to act.

• Be careful when using Frozen Sigil, as it gives a large bounty when destroyed. Only direct it toward enemy heroes when they are too busy to pay attention to it (i.e. when being pursued by your allies). It has a relatively fast speed and can catch up to most enemies.

• Purchase mana boosting items to supplement all of your four active spells, since Tusk is incredibly skill-dependent.

• Walrus Punch deals 4x critical damage when enemies are 50% below their maximum health. Similar to Bounty Hunter's Jinada, it also maims the target to a crawl, so use this to knock out an enemy before they can retaliate. However, this move can accidentally steal the kill due to its potential critical damage, but nevertheless it is acceptable, since Tusk benefits from additional items.

• If you managed to get quite some gold from ganks early on, you could consider going for armor-reducing items like Medallion of Courage or Desolator: they give amplified damage for a reasonable price. Just be sure to use your Medallion on the enemy before utilizing your ultimate on him.

• If you find yourself without mana too often, you could consider going for Arcane Boots or Intelligence-boosting items like Drum of Endurance or Force Staff, which also come with nice abilities. Alternatively, you could buy items that increase your mana regeneration such as Medallion of Courage and Perseverance (which can also be turned into a Battle Fury, should you get other damage-increasing items before finishing the whole item). 

source: http://dota2.gamepedia.com/Tusk