Decelerators were Quantum Lieutenants. Decelerators were instantly recognized for their metallic screech upon entering combat. They would 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 appeared, the player was pretty much in trouble. They would close in, and deal a crippling Slow onto Heroes. Heroes like Wraith, who were naturally slow, should be switched out for a faster Hero. Flak Cannon and Strangling Briars could be used to slow down Decelerators as soon as they appear.
One Decelerator was bad enough, multiple Decelerators was 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 could cut down the Decelerators one by one, switching out to cleanse the Slow. This was also the best strategy if an Elite spawns with any sort of defensive aura.
Blitz was also a good choice. Once you see a Decelerator, you could 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 made the massive Darkspore be obliterated. Surprisingly, Vex could also be used pretty well, if the player's timing was good enough. This applied to all heroes with a teleport attack (including Phantom Charge Necro Variants). Just teleport before they see you, and you were almost guaranteed to kill them before they could even react.
If a team of Darkspore was coming with one of these, take these out first, as it would be much better to run at higher speed during battle.
Using Sentinels against these was pretty much one of the dumbest things to do in Darkspore, since they were already pretty slow. However, with Vex in the squad, this was no longer a problem. Simply Chrono Blink and kill them while they're frozen.