The Walking Mortar
If you've got terrible aim, there's a good chance a majority of these tips aren't going to help you. Fret not, Stormtrooper, we've got you covered. Equip the Hardlight Shield armor ability and the Resupply tactical package to turn yourself into a near-invincible walking mortar. You'll be able to activate the shield to keep you protected while you wade into the middle of the battlefield, picking up grenades from the fallen before heading back behind proper cover to unleash yet another volley of exploding hailstones.
Just be careful of the sneaky ones that get wise to your plan; if someone creeps up behind you, it'll be instant-death even if you've got your shield up. If you notice someone coming on your radar and don't trust yourself to react in time, just throw a bunch of grenades at your feet; at least you'll take them out with you!