Generally speaking, the only proper advantage that the IS has is numbers.
Since Clans have better 3/4 pilots, waaaay more firepower, have more range and, for similar speed, have similar or better durability while you only have numbers, you want to brawl.
In a proper brawl all weapons but small lasers perhaps will be using short range bracket which takes a great deal out of the Clan advantages in range brackets and better pilots. Theres a considerably lower gap between needing to hit 6+ vs. 9+ on a 2d6 than 8+ vs. 11+ or 9+ vs. 12+. Brawling also allows the IS to rotate received damage better(moving damaged meks last and making sure they arent the easiest target, even hiding them etc.) and opens up opportunities to use all the dirty tools and tricks such as battle armor(riding a mech), inferno SRMs, TSM boosted physical attacks, thunder LRMs, even charges and blocking movement. Fast vehicles(hover laser/missile boats like Drillson, J.Edgar, Saracen and such) can also help you growing sheer numbers and can flank and backstab; the enemy needs to take them into account so just their presence alone can open up opportunities. They are low, fast, easy to keep hidden and many of them have space for carrying battle armor right to the clanners' feet. Remember that BA can also spot for indirect fire, and does it at any range and without the shooter suffering spotter movement modifiers(they can jump every turn and still spot!).
To get close you need speed. Your average Clan OPFOR mech unit will be at least 5/8/X, so to stand a chance in closing the gap without taking too much damage you want to keep your entire army at that speed or above it. Dedicated LRM and missile boats may be excluded from that rule but they too should jump, and you shouldnt use too many of them.
One good Clan buster mek is the Draconis Combine's WVR-7K: yes its kinda fragile with XL engine, but its cheap-ish, has 5/8/5 movement and its weaponry is all pulse lasers and streak SRMs. Theres many more and lots of slower mechs too can be useful way or another with the right support, or them being the support themselves.