As has been said before, it's not what you bring, it's how you use it. There's plenty of lov-BV stuff with hideous possibilities. Make sure to take plenty of weapons not normally seen on a battlefield. Some suggestions:
Infernos (as has been stated above)
Swarm LRMs (Hot-Loaded too, they won't expect to see lances of Archers and Catapults attempting direct urban combat)
Go out of your way to kill pilots. Use punches rather than kicks.
Unusual physical attacks. Push their 'Mechs into burning buildings, minefields, quicksand, Tactical Operations has a lot of options.
The Star League was the heyday of the IndustrialMech. It shouldn't be too hard to inflate your force with low-BV SRM, AC, or RCL Mods.
Don't ignore the basics, like wrecking balls. Fluid guns have essentially limitless options (I don't see why you couldn't fill one up at a local butchery. Just use the paint ammo rules with a morale check attached.)
Following the above, the Heavy Industrial Equipment section is your best friend. Fill Dumpers with infantry, bombs, rocks, or bodies. Maybe have some 10-ton cargo containers made with weapon mounts for your LoaderMechs. Use lift hoists to take hostage civilian vehicles.
If you have more than one player, use ECM to disrupt them. Maybe they have to have LOS not interrupted by ECM (or a really good Comms roll) to talk to each other. Gets even better when you use SLDF schemes too, so they have a hard time even with visuals.
Pull out some big, bad, experimental Amaris goodie they haven't seen before. He was into one-shot wonders, maybe pull out a prototype Kraken/Behemoth, or some giant custom flamer-spammer with double heat sinks. Use at least one Firestarter.
Make sure to have the Amaris troops taunt them, gloat over kills, threaten hostages, etc. Maybe even give them a chance to rescue a couple, like take an aimed shot with their Gauss Rifle against the motionless Atlas holding the school bus hostage...then, on a fall direction roll of 1, the bus gets crushed anyway. >:D