If you can find a mech with low accuracy, either due to short ranges (an old Hunchback would be perfect) low skill (green troops are easy pray, or IS regulars) or problematic weapons (some mix of MRMs and HLs is a good start [I actualy did a mech like that once, and it wasn't as bad as it sounds]) you can just run circles around it forever untill your ML scores a telling hit. It is after all really, really fast, so it can dictate the range and pace of pretty much any engagment, and if the terrain is right and there's a good mix of cover and room to run you can do a lot to make up for your lack of armor. Given enough time, you can pull out a win.
The key problem is, an ER ML hasn't got enough range to keep Clan opponent at bay. Since most likely opponents can hit at least 5/8, if the Icestorm is in range its opponent can most likely close into medium range, or else fall back out of ER ML range, if they win int. A mech with big guns (and what Clan mech lacks them?) will be able to get into short range some times, and that's just the end for an Icestorm.
Plus, the Clans have a lot of access to good accuracy, through skill, pulse lasers, TCs, and just generaly good range (an ER LL is automaticly in medium range if the ER ML is in range at all, and its only seven hexes to short range). The only mechs that don't have thouse things tend to have the speed to chance the squerly bugger down and take it to task (I'd actualy never take one against a Fire Moth, since the Moth can easily close the range up and demonstrate exactly how many more guns it has in a very in your face way).
Some of the old 3060 mechs are suitably slow, as are some (but hardly all, or even most) old SL era mechs, which are probably best bets.
Honestly, I'd take a hard look at Pack Zell if I was forced to make much use of this thing.