As pointed out, there are many ways of dealing with an Assault 'Mech, at least if you have a significant speed advantage. I've put down Assaults with a Locust (1-vs-1) on several occasions, although the Locust loses more often than not. If you've got several Light 'Mechs against one Assault, speed modifiers, firing angles, and unanswered kicks from behind (or punches, if you weaken the rear torso armor first), give the Lights a significant advantage over the same tonnage of Assault.
MOST assaults have a "weak side", where their arm-mounted weapons are less effective (often due to minimum range modifiers) than the other side, so while one Light parks directly behind, a second Light sits one hex further behind and to the side where only the worse arm can fire at it, but it can still fire into the Assault's rear arc. Note that a couple of Assaults (Stalker, Goliath) cannot torso-twist, so you can sit in a side angle where only one arm can fire (the Goliath doesn't have arms to do so), while the Stalker could flip its arms backwards and deliver fire from BOTH into its rear arc, but NOT into a side angle. The Banshee has no weapon in the one arm, so it has a glaring blind spot off to that side and back from its rear facing.
Dealing with TWO or more Assaults is a much bigger problem, since one can protect the other's back and torso-twist to pour its full firepower into anything that tries for a back shot on its teammate. At that point, you need to go for movement modifiers and sniping at range, preferably with a long-range energy weapon such as a LL or PPC, where the Assaults need 12+ or 13+ to hit you, while your own shots are at 10+ or 11+. Unfortunately, those matches are no fun to play out, because it can take 20-40 turns for those poor-odds shots to add up enough to take down an Assault.
The other option is SRM spam on a massive scale. With enough SRMs, the through-armor critical hits and head shots will frequently take out those Assaults even before their armor is breached in any location. Again, using fast units to keep the movement modifiers up is a big help. The Javelin is a good 'Mech for the task at 6/9/6 with twin SRM-6 racks, and the SRM version of the Trebuchet will generally suffice, although it's only a 5/8 mover and is a bit light on the armor protection. An SRM Carrier probably won't survive more than one turn with a line of sight to an Assault 'Mech, but by taking advantage of terrain to get it into short range, it has the potential to destroy or disable any 'Mech in a single round of fire.
Going up against an Assault with a bunch of low-speed Mediums plays to the strengths of the Assault, and about all you can do is try to make sure that any time one of yours has a line of sight to the target, they ALL can fire. Hiding one or more while the Assault gets to overmatch the others is a good way to lose.