I love the ice ball. I've upgraded it 3 times, once for more health, once for more damage and the last upgrade was so that it uses less rage. So my ice ball only costs 20 rage with those 2 upgrades. I had a shot once where it rolled across the entire length of the battlefield and it critted for over 3k. Took that stack of giants right out.
It is also useful for distracting enemies or keeping it next range units. It has an insane defense, so it takes a long time to kill it.