You melee with
. You can also jump with
and hold
to do a ground slam, which also deals area of effect damage.
Rank 2 of the Combat Tools skill in the Combat skill tree, will increase melee damage to enemies by approx. 3%. Likewise, unlocking and using the Vanguard profile, will increase melee damage %, starting at 20% with level 1, increasing by 5% per level.
These may help in obtaining this quicker as the area of effect damage on the ground slam can become quite devastating with the proper increases in melee damage.