Ice beasts are nearly invunerable NPC enemies which were designed to instantly kill any player regardless of their vita level. Normally, the only way to defeat them is to trick them into walking into the lava section of the only map they normally spawn on. They are part of the "Frost sabre" quest.
Trivia: When Grin was hacked, his Set Reactor spell was used to summon a large amount of Ice beasts into Buya. Prior to this, it was believed Ice beasts were completely impossible for players to kill without the use of lava, but it was found that by exploiting the fact they "lock on" to the target of a specific character and will not change that target unless that character dies, Ice beasts were defeatable by tricking the Ice beast into entering an area they could never reach their target at.
With the exception of Doze, it is impossible to cast any spell on an Ice Beast which is not a stat-based attack (such as Hellfire or Whirlwind), making it incredibly difficult to defeat them, given that curses cannot be cast and that even Doze will not stop them from moving. They also possess Vita levels that exceed 18 million vita points.
This means that even by today's standards, Ice beasts pose a great deal of a challenge to even the strongest characters in the game.