Sounds like 90% of the TW or AS games I've played in the last...over a decade, now, with the addition that you also never knew beforehand the faction, tech level, or even era of your opponent's force. :)
It should be noted that a lot of the weapons that are effective at countering a Cannonshop or Me-style force are also effective at hurting mechs, albeit sometimes in different ways(plasma, for example).
With that in mind, there's not much reason to NOT prepare as if you're going to face a conventional-heavy force.
we always treated it as 'part of the challenge", you roll with the army you've got, rather than an army tailored to a specific situation. FSM changed things a LOT-because it created a situation where the optimal solution if you actually wanted to play, was to dump everything into as few units as possible and hope for a duel.
which gets boring quick, since both sides end up in Clan energy-boat-assaults parked at 'medium' rolling dice until someone falls over.
Our group dumped FSM from BV calcs as it took the bulk of the actual 'fun' out of the game, and moved to using a change to the initiative rules instead, setting the initiative order to a 'front loaded' system that inherently limits 'initiative sinking' effects for asymmetrically numbered forces.
(For some reason, this still wound up with roughly even chances of victory-when the smaller, and usually heavier/more advanced forces were played right. I've lost more than a few weekend fights while still
tehnically outnumbering the other side. esp getting sloppy with my tactics.)
but the point is, massed LBX isn't a bad way to go, if you have the dice to do it. I'll echo others in that the transparent 'fishingtackle' box with 2 dice in each compartment, shaken-shaken-slam! (shake multiple times, slam on the table to drop 'edges' onto flats) is a great way to handle massed hit locations, and a nice little 'dice tower' for your to-hits or PSR's are nice additions, and can be a real boon for speeding up turns, even in lance/company battles or fights where you've got lots of weapons firing instead of just one.
We used to have a display-board with the hit location chart and cluster charts displayed at large scale, just so that it was easier to quick-reference for new players and spectators when Dave was doing 'gamestore' matches for his Commando credit.
With all the supplementary charts in TW now, I think we'd need a bigger display board.