It's certainly possible to win against the clans in AtB. It is not easy however, you kind of need to fight dirty. Their range, and speed make it really difficult to fight them.
They are usually pretty crunchy once you start actually doing damage to them. The XL engines in most of their mechs mean I get a LOT of engine kills once crits start flying around. Their mechs carry ammo in all sorts of places too so I get a lot of ammo explosions that rattle their pilots until they are mush or straight-up mission kill the mech.
So, you need to work to reduce their range and speed advantages.
First, figure out ways to block their line of sight. Smoke (or infernos) or lighting terrain on fire helps. If you can cut the engagement envelope from 20+ hexes (suicide for IS mechs) down to 12 or so hexes, it'll make your weapons hit far more reliably. It is also good for what I call "splitting the combat." Use smoke screens (via smoke munitions or ignited forests/buildings/rubble) to break engagement with a number of enemy mechs so that you can engage the rest piecemeal. (This works well with broken terrain tip below, and makes recon attacks REALLY easy)
Second, you gotta slow them down somehow. Bring some mines! Those repeated hits to the legs add up, the bot usually just walks right through them to engage. You'll thank me once you close for physical combat!
Also try really hard to choose areas that force the clans to run through broken terrain but at the same time keeps your area relatively clear. This isn't always possible of course...
I've had much better luck with heavily armored and armored bricks as mechs that stand and deliver than I have had with mobile striker mechs that sacrifice armor or weapons for speed. Unfortunately with pulse lasers, targeting computers really light LRM racks without minimums and 1/2 pilots, they are pretty much just going to hit regardless of TMM, so might as well pack extra armor.
I always try to bring a reinforcement lance on every mission. I'll usually have a 5/8/5 lance of Quickdraws, Wolverines, and brawler Griffins (modded to add a TAG laser somewhere) in addition to my light lance that I use only for reinforcements. I pray that 1d6 rolls my way for one or both.
Other than that, guided artillery is seriously your friend, maybe try to squeeze in a modded Arrow IV hetzer to your lances somehow.
Depending on the rules set, (and IIRC) one type of infantry unit gives you the ability to just remove a scenario, that can help when you see one of those really stacked missions.
I still lose, a lot. But with some tricks you can get some really nice clantech salvage even if you lose a whole contract sometimes. I can usually grind out a win as the scenarios are rolled, but every once and a while too I'll either find myself removing the opponent's reinforcements or toning down their forces somehow. (swapping down a mech's weight class or adjusting the g/p of an enemy pilot)
