en_tn_condensed/2sa/17/12.md

come on him

This means to purposefully go to where he is and then to attack.

we will cover him as the dew falls on the ground

"we will overwhelm and completely defeat David's army"

dew

the fog or misty cloud of water that can fall onto the ground during the night, covering the ground and leaving it wet in the morning

We will not leave even one of his men ... alive

"We will kill every one of his men"

him himself

"David himself"