Hard to argue with the Goshawk, at least the standard version. Jump jets, pulse lasers, and a targeting computer, with some SRMs for crit-seeking? Perfection, if a bit cheesy. (Dump that MG ammo, son!). But hey, it's a Grand Melee, cheese is encouraged!
...that said, if you have a player who will only use assault Mechs and nothing else, that sucks- adapt to the game or don't play, dude. I used to be the same way when I first started out- if it was under 85 tons it was crap. Now, I'm a jaded Jade Falcon who doesn't use assaults unless forced into it in-general, preferring mediums and fast heavies. You learn new tricks as you play the game- but you have to actually play other units to learn them. (Actually, a Goshawk getting behind an assault Mech is pretty entertaining. You're fast and jump-capable, so you can almost always either get back there to munch on that sweet back-armor, or hop to safety if you need to.)