When several opponents target you at once or you are in danger of dieing use it immediately. You can be healed while blocked so call out for healing if you really need it. You can also use vindictus gold Ice Block to remove any debuffs or DOTs on you but only do this when absolutely necessary.
Ice Block is better as an emergency cheat death button than a self cleanse. Besides Ice Block there are two other sheilds you can use.
Ice Barrier and Mana Shield are two abilities that will absorb any incoming damage but need to be used carefully. Ice Barrier is useful, but has a long cool-down. Mana Shield has no cool-down, but will eat up your mana.
Use Ice Barrier when possible, but only use mana shield when you have no other option. As a final option, always try to use the terrain to your advantage. Most arena's have pillars or ramps for you to hide behind.
The main advantage of this is that the enemy will not be able to attack you, especially at range. But you will also be protected because the enemy will not know where you are. Strafe in and out of your hiding place to reapply spells and use instant casts to attack.
Focus all your efforts on preventing the healer from healing. If you can, also use rank one frost bolts on any of the enemy melee to keep them from getting into range as quickly. This is especially true for mortal strike warriors.