You'd be lucky to get an assault on a tervigon - they ought to be shielded with pretty upgraded gaunts...
counterattacking poison 4+ gaunts can really break assaulting marine groups - ok, they'll take losses, but if they kill half of you, its worth it (pretty likely from assaulting 30 gaunts - with the upgrades thats around 15 saves to make). With 5 tervi's i'd be suprised if some werent carrying catalyst, so about 1/2 of those fearless losses wouldnt be happening. Your big assault team just got stuck into melee with gaunts that can beat them down eventually, but take a long time dying. pretty good use for 150 points worth of gaunts.
The way to beat this list is in shooting. Tervigon heavy lists spend a lot of points on termagaunts and a few slow relatively ineffective MC's.
Just tank up (landraider or two, rhinos/razorbacks, something artillery based), and use big templates (irrelevant of strength, but s6+ is good) to do damage to the swarm. Focus your fire on anything that stands a chance of breaking tanks... hive guard, zoans, tyrannofex, and trygons spring to mind. A land raider with flamers is your friend here, along with one with lots of lascannons on it to kill the big stuff quickly.
Tervigon only has a 3+ save when it comes to it, so throw some wounds its way and they'll break pretty quickly - its reliant on T6 to keep it alive, so throw high str stuff its way. Power weapons if things are somehow getting desperate, but if you get out of your tanks to kill something like that, write that squad off - it'll get mobbed by the bored termagaunts.
Focusing your fire from inside your tanks until the anti-tank is gone is the way to win this.