You need to hit enemy players 15 times with your 2 ability; Mummify.
This should come naturally and can be done in all modes.
If you're having a hard time hitting enemies with it you can try hitting them first with your 3 ability; Grasping Hands, which will sow them down or you can try just running up to enemies and hitting them with your mummy wrap at point blank. You can also try using this on enemies that your allies have already stunned.
Having some cooldown reduction (CDR) items may help with you getting to use this ability more frequently.