The volushomeworld, Irune, has a high-pressure atmosphere that supports an ammonia-based biochemistry. To survive on other planets, the volus must wear pressure suits and breathers or, when facing combat, specially sealed armor.
Currently, the Volus Mercenary Sentinel is the only character in the game to lack any direct offensive powers, meaning his only forms of inflicting damage are through his weapon and Combat Drone (Decoy can also damage if evolved for Shock and/or Exploding Decoy).
Since this character is less power dependent than the other volus, carrying slightly heavy/powerful weaponry can be useful as the player can't depend solely on Combat Drone/Decoy to inflict damage.
The volus light melee is a Tactical Cloak that lasts for around 3 to 4 seconds. It does not share a cooldown with the normal powers, but instead has its own separate cooldown (around 4 seconds).
The volus heavy melee generates a sort of barrier around the volus, giving him minor damage reduction and repelling weak enemies. This barrier does inflict minor damage to enemies that touch it.
It's advisable for volus characters to put all fitness upgrades into shields and health, as the volus heavy melee is the only means of dealing melee damage, and its damage is relatively weak even with bonuses.
Since the volus' height is around half the height of a regular human character, they cannot take cover normally, but can take "soft cover" by simply walking into it.
The Volus Sentinel's role in battle is pure support, using Decoy and Combat Drone to distract and/or lure enemies away from the squad, while keeping a low profile thanks to Tactical Cloak and his small size.
Combining an area-of-effect weapon, like the Striker Assault Rifle or Scorpion, with Decoy's Shock evolution and Combat Drone's Shock and Chain Lightning evolutions can allow you to indefinitely stagger and lock down large groups of enemies while your teammates finish them off unimpeded.
Shield Boost makes the volus a better medic than the Geth Engineer: while Geth Turret recharges one ally's shield at medium range, Shield Boost recharges all allies' shields around the volus.
Properly specced, this ability can restore ~1100 shields, plus ~220 per second for 3 seconds. The final evolution of this ability further increases damage reduction, giving all affected players 50% damage reduction for 6 seconds.
As well as this, Shield Boost makes you invulnerable for a second. This can give you the time you need to revive a team mate, complete an objective, or buy your team enough time to endure the worst of what the enemy can throw at you.
It would be wise to equip the Acolyte, since the Volus lacks any real powers to battle against any form of protection. The Acolyte will at least give you some means of stripping shields or barriers from Phantoms, Atlases, Nemeses, or Centurions.
Decoy will adequately distract the smaller troops and keep your allies from going down, but harder-hitting foes will often destroy your decoy very quickly, like Atlases, Turrets and Phantoms.
Combat Drones upgraded with Rockets will be less effective against Guardians since rockets will only stagger said foes, it will not expose them. Put the Drone behind them to counter-act this.
Consider carrying an Acolyte and (if you can take the extra weight) an anti-armor gun. You will need them since all enemies have Health, Armor, and/or Barriers.
Decoys will not prevent Abominations from heading towards you, but they can distract the Scions or Praetorians, giving your precious seconds to return fire, or retreat.
Combat Drones can kill Seeker Swarms if they are killed and the enemies is in the process of summoning said swarms. This will give you a little more protection than other classes, but nonetheless you should still be careful, as you may find yourself unable to cast powers if you don't stay on your toes.
Shield Boost will be invaluable here, due to the collectors mixed array of quick and light, and slow and heavy attacks. The Praetorians especially will make light work of you due to your very weak levels of endurance.
Equipping the Acolyte will allow you to deal with Geth easily enough since every single enemy (except the basic troopers) has at least a layer of shielding to protect themselves. Said weapon will quickly solve that problem and expose them to a shocking from your drone, or a battering from your team.
Decoy can be really helpful in detecting Geth Hunters in the vicinity of the Decoy. Upon destruction it can adequately deal with their shielding and allow you to dispose of the Hunters more easily.
Combat Drones will be less effective here since most of the enemies are hard hitting specialists and will quickly deal with your Drone in just 1 or 2 quick shots from a shotgun or rocket.
You should equip yourself an Anti-Armor weapon here because only 2 enemies have shields or barriers. The Paladin, Talon, or Carnifex all fit the description and will deal with the Reapers nicely.
Combat Drones will help keep Brutes off you if done correctly, due to the fact they may need to turn to hit your Drone.
Also, the Decoy is less effective, as of an update which made it so melee enemies will completely ignore the Decoy, and instead focus their attacks on you.
Your biggest problem will be the Ravagers, not just for their powerful cannons, but for their swarmers too. They will quickly chew through your shielding and then eat away at what little health you have if you don't stay alert. On top of that the Ravagers also leave pools of acid which will also harm you and this can spell your doom if your powers are on cooldown. Combat Drones or Decoys will partially alleviate this problem, but their attacks can do more than just annoy.
Consider carrying an anti-armor gun to deal with Ravagers.