I got this early in the game with the Trickster, using a mixture of Temporal Blade (
or
depending on where you map it) and normal melee (
). Basically I just went up to enemies and used these skills whenever possible. Temporal Blade has a cool down period depending on your level and normal melee can be spammed a few times at one go.
When there's a crowd, I used Slow Trap (
+
) to slow them and just keep spamming
. I got the 50 kills before I reached the Solar Tower.
With the Trickster, you also unlock a skill at Level 4 that allows you to teleport to the back of an enemy. You can shoot a few times to reduce the enemy's HP, teleport to the back and
him for a kill to count too.