From my experience playing with and against Dengaroo it comes down to two questions:
Do I have enough firepower to kill Dengar and still have enough left to kill Manaroo? HLC Brobots are actually pretty solid against Dengaroo, they have the firepower to take him down and can out maneuver him in close. If one dies that is fine, Manaroo vs a healthy Aggressor is fine by me. If I have enough raw dice to potentially kill him and only trade one ship I'll do it.
Do I have the speed/tools to catch Manaroo? Fast ships and conner nets come to mind. If I can kill her before I take too much damage Dengar is a lot less scary.
As always it really comes down to how good the individual players are with their respective lists and how much practice you've had.