This build’s only downsides are its squishiness and heavy reliability on Spirit.
Skeletal Snipers deal great damage though, especially via their Gas Arrows.
Detonate Dead
Detonate Dead works perfectly with the scorpion minions summoned by Unearth.

This means Flame Wall becomes all the better, and the ranks of your minion army continue to increase!
Vulnerability
Vulnerability curses all nearby targets and makes hits against them ignore a portion of their armour.
It’s a good support gem for any skeletal minion, honestly, and works well with Last Gasp.

Use it with Pain Offering to make your bone spike stick around beyond its typical expiration date.
Ablation
Ablation’s another possible support gem for Pain Offering, though it’s riskier than Persistence.
It sacrifices a portion of your health to increase the effects of a buff.

Magnified Effect
Magnified Effect makes any AoE skill larger.
Fortress
Fortress takes any spells that generate in a line and switches this up to a circle.
This is a great way to supplement the strength of your little Ghost Rider-esque buddies.