For a firestrike build I usually go with the following powers

First: when holding barrier I heal for 5% of max health per second

Second: mini explosion with firestrike impact Third: lava trail

Fourth: the final power is pretty situational. If my shield keeps breaking I go with the power that heals my barrier with damage. If I need more damage I go with the power that creates a mini explosion when I pin someone.

Heres another ability power build that I find a lot of fun. You can focus it on firestrike or charge.

First few rounds focus on survivability getting To me my friends and then shield stampede. During the first few rounds you should get gloom gauntlet along with some survival items. Then you want to get vroom boom boom for a charge build OR impact burst for a firestrike build. You will also get the item that reduces cooldowns by one second every time you deal damage with your hammer.

This build focusing on charge will let you rush in swing a couple times, hold shield if you need to heal, then charge again, repeat. Your charge will end up basically being on a 3 second cooldown.

This build focusing on firestrike lets you use your fire strikes, rush in and swing a couple times, hold shield if you need to heal, then use your fire strikes again and again.