Currently Fragger will win just about every one on one against another mercenary because of the damage output of his gun, as well as his higher HP.
The primary difference between the Fragger class and the other classes is his ability to throw grenades, which should be the primary focus of the class. Removing the ability to cook grenades is not the right route to take. I think that in order to balance the Fragger class, a couple of things need to be done.
1. Reduce the damage on the Fragger's default gun. This is one of the main reasons why he wins most one on ones. He already has the most HP out of all the current mercenaries, he doesn't need higher damage too.
2. Remove the ability to cancel cooking a grenade by switching weapons.
3. Higher cool down on grenades. This will make smart players save their grenades for groups of players just off respawn, rather than winning a one on one.
4. Reduce the grenade count to 1 grenade, further promoting saving your grenade for a time when it has a bigger outcome on the match.
I also want to touch on another thing: Bigger maps (example: Dome) will make Fragger less effective. Obviously, there are certain points where Fragger is definitely helpful, but slow movement speed and far lines of sight make Fragger and his grenades less effective.
These are just my opinions. Respond with what you think.