There are various ways to increase your elemental damage. Here is the short guide how to.
1. ADD all stats, every character gain Elemental Damage from STR/AGI/VIT/ENE
2. Find a good Pentagram with high damage!
3. Upgrade your Pentagram to +15! then to Mastery Pentagram
4. Add Radiance to unlock Punish(PVM) Only and Mastery Errtels to Unlock Pentagram SET options.
5. Add Elemental option to your 4th wings or Pure Wing Core to your 5th wings.
6. Find and Upgrade your Guardian Mount - Horse -> Leviathan
7. Elves can also buff you with additional Elemental Damage!
8. And remember that you deal different damage to different elements in PVM.