The key to combat with the LV-426 xenoform is to avoid CQB. As their prowess apparently lies in the purely biological realm, they are quite vunerable to stand-off and ranged weapons. The standard UD-4L Utility Dropship carries an array of some 57 air-to-ground missles and rockets, including the M598 150mm Banshee 70 "beehive" round, and an internal 25mm cannon.
After all, the nasty buggers can't fly.
Importing from other venues:
A Bolo's Hellbore would be a powerful "nest-killer", with infinite repeaters and particle shielding to repell boarding attacks. The acidic nature of the xenoform corpses might be an impairment to the tracks and road wheels of early Marks without flight capability, however.
The various armoured mecha from the "Warstrider" novels that proved fairly effective against their own biotech alien hostiles. Their nanotech armour could prove highly desirable.
The Panzertruppen from the "Hammer's Slammers" novels offer a good balance of firepower and protection. Powerguns might be good performers against the xenoforms, preventing the acid contaimination that marrs the use of projectile weapons.
To be really, really sure, the Imperial Navy's Death Star Mk.II or the Vorlon Planet Killer come to mind.
By the way, I can highly recommend to those interested in this subject the "Aliens: Colonial Marines Technical Manual" by Lee Brimmicombe Wood. Great mechanical, tactical and biological data. The first three novelizations of the movies weren't too bad, either.