If Sent/Mara are rock, then Agent is paper. If they catch a Sent out of stealth and stun them, they are going to die 90% of the time.
Sents are not invicible, just learn to play your class better.
There are, at best, two classes that can reliably counter the Marauder/Sentinel: Snipers/Gunslingers are perfect for it with Leg Shot, knockback, stun, flash grenade etc., and the 23/1/17 hybrid Assassins/Shadows who have the damage and the survivability and the control to cope with them. If they play their class really, really well.
Concealment Operatives have nothing on Sentinels. Knockdown for 1.5s out of stealth, stun after that; Sentinel uses stun breaker and then wrecks the Operative's face in seconds.