Detonate dead. How to increase its dmg with support gems?

Probably the first two gems you want to combine with detonate dead is

1) Concentrated effect
2) Increased AoE

I would then do something such as....

3) Added Chaos/Added Lightning/Culling Strike

If you are cursing Fire Penetration isn't that important.

If you have no AoE nodes Conc effect is probably not that good since the range isn't that big to start with.

I'd go with Increased AoE, Culling Strike and maybe Added Lightning damage for the last one (even better Added Chaos damage but it's a bit rare).