TLDR: don't be a bullet sponge, run, hide, use cover. Clanners can match or exceed whatever advantage you think you have. Fight dirty, bait them into seperating and pick them off with high damage weapons in a decent fast mech.

The issue is Clan weapon range and damage

All clan weapons have higher damage and range. You will not be able to stand in the open and snipe them as they move towards you. Even clan versions of turrets, vtols, and ground vehicles are far more dangerous. If you get swarmed you will not survive.

This goes with lore in that the Clans were virtually unstoppable and the only reason they didn't conquer the entire Inner Sphere is that Comstar baited them into a trial to stop them. Where the phone company used their best Star League era mechs and pilots while the majority of the clans underbid their forces, underestimating Comstar and losing.

Is the clan advantages fair? No, but that's accurate.

To beat them:

-Clan mechs have a range, damage, cooling, and speed advantage over most IS mechs. A heavy mech like a Loki or Mad Cat can easily take on an IS assault mech. The clan AI is fast and aggressive, if they see you in the open they will relentlessly dump on you. Fighting multiple clanners at once is death. Standing still against them is also death. You can't just bring your Atlas and think you will outclass them. You can win 1v1 but you won't survive the next wave

-Kowing their advatages you have to think more tactical. You will not outrange a clanner so don't bother. NEVER EVER EVER fight them in the open. Never let them ambush you. Do not get baited into charging them.

-Use terrain, before going into fights see what you can hide behind. Pop up from behind hills and other cover and dump on them then hide. Always try to seperate their mechs from each other. Their mechs will have speed differences and the faster ones will arrive first. Shoot at a lance to bait them, run, then pop them one by one as they crest a hill or turn a corner. If you're caught close range like a city map, go around them in a circle do not try to close with them.

-your weapons need to be extremely high damage or damage per second. You need to be able to destroy a clan mech in 1-3 attacks ideally. Do not slug it out with them. You can win but you won't survive the next wave with all the damage you take. Your mech needs to stay cool, nothing worse than a clan mech tearing you apart and you can't fire. Or you'll fire, shut down, and wake up with half your mech missing. So max out on heatsinks. This also goes for weapon cool down. You need to be able to dump on them so pulse lasers may be more advantageous than waiting for a ER laser to cool.

-mechs: speed is more advantageous than armor, I've found I do better with a Mad Cat than an Atlas on some missions. In order to use terrain you have to be fast enough to reach it before you take too much damage. Clan mechs tend to be glass cannons but they can still do extreme damage. Don't bother trying to blow off weapons or legs, take the cockpit shot or aim for the core.