Use Dash Strike kick to kill 10 enemies.

'Dash' is a Nanotrite skill that you will unlock during the early missions of the main story. You press 'LB' and a direction to quickly dash away from incoming enemy fire.

Once you have 'Dash' you will now need to upgrade it in the menu to at least level 2 so that you can then purchase a perk for it in the second tier, the perk being 'Dash Strike', so purchase this when you can.

Now you simply need to use it against enemies. As per usual with grinding, Mutant Bash TV is ideal for this, be sure to do the normal challenge as the specific challenges remove your skills. Now you can use 'Dash Strike' ('LB' + 'RS') to do a front kick that kills the mutants on easy in 1 hit. Repeat as necessary.