This article gives The Arbiter usage tips.
To maximize its effectiveness, equip The Arbiter with:
2 Area Mods.
The Ammo Mods double the ammo slots from 8 to 16. The Aiming Mods make sure that the Rockets you fire will get a better Aim at an enemy that is far away. The Impact Mods make sure that even larger enemies will be knocked out for a few seconds, allowing you to get another shot at them. If you have The Silencer, you can put 3 Aiming Mods and 1 Impact Mod, so all 3 rockets that you fire aim at the selected targets. The Area Mods are there so when the rocket hits the enemy, it also hits nearby enemies. Speed Mods are not necessary because equipping one or two only increases the rate of fire very slightly, and the other mods are needed. Perhaps you may use a spread of (4 Ammo Mods, 4 Speed Mods, 1 Impact Mod, and 1 Area Mod), if you do not care about aiming, and rather have a faster rate of fire.
The best mod to use on The Arbiter is the Time-Bomb Mod. Instead of hitting the enemy directly, it left an electrical sticky bomb on them, that exploded violently about a second later. This violent explosion turns the screen a negative color for a split second and the electric time bomb also damages or kills nearby enemies. This is especially deadly if The Silencer went up close to an enemy and shot all 3 rockets, creating a triple explosion.
If you do not have access to the Time-Bomb Mod, you can always use the Mini-Bomb Mod. The Mini-Bomb Mod creates small bomblets after the rocket hits an enemy. This can be useful because it can provide extra damage on an enemy, or it can hit surrounding enemies or swarms.