Decelerators are Quantum Lieutenants. Decelerator are instantly recognized for their metallic screech upon entering combat. They will dramatically slow down Heroes, then attack with their blade-like feet.
Traveling from world to world dominated by the Darkspore horde, decelerators are terrors to those who oppose them. No matter how agile an attacker may be, a decelerator can drastically slow that attacker's attacks, defenses and even retreats. To defeat a decelerator, the best hope is to get as close as possible, as quickly as possible--teleportation being the safest way.
As soon as a Decelerator appears, the player is pretty much in trouble. They will close in, and deal a crippling Slow onto Heroes. Heroes like Wraith, who are naturally slow, should be switched out for a faster Hero. Flak Cannon and Strangling Briars can be used to slow down Decelerators as soon as they appear.
One Decelerator is bad enough, multiple Decelerators is downright terrifying. The best thing to do would be to keep your distance, firing when you can, dealing debuffs when you can. Alternatively, melee Ravagers can cut down the Decelerators one by one, switching out to cleanse the Slow. This is also the best strategy if an Elite spawns with any sort of defensive aura.
Blitz is also a good choice. Once you see a Decelerator, you can use Electron Sphere, to take it from a distance. Then use Plasma Wreath and combine it with Ride The Lightning to teleport to the Lieutenant. The effect would render the Decelerator damaged and shocked and adding the Plasma Wreath plus a few hits makes the massive Darkspore to be obliterated. Surprisingly, Vex can also be used pretty well, if the player's timing is good enough. This applies to all heroes with a teleport attack (including Phantom Charge Necro Variants). Just teleport before they see you, and you're almost guaranteed to kill them before they even react.
If a team of Darkspore is coming with one of these, take these out first, as it'll be much better to run at higher speed during battle.
Using Sentinels against these is pretty much one of the dumbest things to do in Darkspore, since they're already pretty slow. However, with Vex in the squad, this is no longer a problem. Simply Chrono Blink and kill them while they're frozen.