That's the point. If you want to jump in and get in peoples faces, you don't play a merc. Simple as that. Mercs stay back and support their teammates from range, as he was doing. The only problem with mercs is that they have inadequate tools to defend themselves once people start focusing them. That is what needs a buff, not the damage.
The reason they're squishy up close is because they do massive ranged damage. They don't need to be tanky. They're a ranged class.