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 and Mastery Errtels to Unlock Pentagram SET options.
5. Add Elemental Options to your level 4 wings!
6. Find and Upgrade your Guardian Mount - Horse -> Twin Tail
7. Elves can also buff you with additional Elemental Damage!
8. And remember that you deal different damage to different elements in PVM.