I haven't used the Thunder Hawk much since I don't yet have a mini for it. Now I'm wondering: how much BV should a player dedicate for a guard force around a Thunder Hawk? Maybe a pair of lights, or an assault tank, or a bunch of battle armor? Is it even worth it? Perhaps a large tank with a short-range knockout gun to deter units trying to flank the Thunder Hawk.
Well, first off, you have the right idea on how to use one- with heavy support. It's your queen in chess- it can do a lot of great things, but losing it is crippling. The problem is that it's pretty easy to lose a Thunder Hawk- with so many ways to go 'boom', and a big engine that doesn't react well to 'boom' events, it's a surprisingly fragile machine. So proper support is important.
I for one like to make sure my bases are covered. The rifles aren't as useful in-close against the kind of fast targets that will attempt to bum-rush your Hawk, and it really should be focusing on using the rifles to deal with longer-range targets anyway of the heavy and assault weight varieties, not wasting shots on fast-movers. A Hunchback makes for a great bodyguard, as they have for centuries- anyone who gets close enough to start irritating your crown jewel gets an AC/20 to the face (or whatever iteration of that gun your Hunchie of choice brought to the party). Even a fellow 100-ton beast coming for your Hawk can't ignore the Hunchback's firepower- but losing a Hunchback is probably worth the sacrifice if it means your Thunder Hawk can withdraw safely.
Another good choice is the Battle Hawk. A big problem for the Thunder Hawk is fast-moving harassers like the Spider and... er, Harasser, units that speed in, deliver a modest punch to the rear of a big Mech, and scoot away. A few passes by units like that can cripple or kill a Thunder Hawk, but it's not really all that well equipped to deal with that threat (do you want to waste your Gauss ammo on a Pegasus instead of an Orion or something like that?). The Battle Hawk's pulse lasers make for a very accurate and powerful close-range smack to the face of such units, freeing the big guy to keep throwing Gauss slugs at the enemy until it stops being funny.
Thunder Hawks are priority targets for the enemy (can't imagine why...), and so they're vulnerable to the enemy's air power- if you can get a good bombing pass from a fighter in on that thing, you can get rid of it pretty easily. Units that excel at AA, as a result, are a good thing- so units with LB-10X batteries are always welcome. The Gurteltier is a little too slow to keep up with the Thunder Hawk in any but flat terrain, but if you're setting the Hawk up on a hill or other strategic area putting a couple of Gurties nearby is handy. And hey, if the enemy doesn't throw planes at you, LB-10X cluster shots are a great way to exploit the holes punched by the Gauss rifles anyway, right? Another Steiner-flavored LB-10X unit to consider is the variant of the Hollander, which is a little underwhelming on its own but makes for an excellent support unit in a case like this.
I'll be honest though. The best way to protect a Thunder Hawk is to get rid of the damned thing and get a couple of Alacorns instead.