It certainly looks winnable by the PC's, but there's a lot we don't know, such as size of map, map features, and objectives. And of course, an early cockpit kill to a PC would change the balance tremendously. We also don't know what tactics your players used. I think I would've gone for something like this:
Turn 1: Summoner prime pilot challenges Warhammer IIC to combat
Turn 2: Timber Wolf pilot challenges Summoner D, Woodsman attacks static laser emplacements unless Stinger challenges him
Turn 3: Cauldron-born and Woodsman commence cleanup on clan support forces, looking to place themselves into good ambush position on either the Warhammer or the Summoner, but not obviously so. This should go on for a few turns until the perfect opportunity presents itself to...
Turn ?: ...
break Zell and crush one of those two mechsIf by some chance, the clan pilots challenge first, you can try and taunt them into accepting a different challenge, OR you can just go with it and do your best. It's really about keeping Zell going until the perfect timing presents itself, such as support mechs in the back arc of the target, or the target stays stationary for a turn, or whatever, but regardless you should be in position to break Zell within a few turns. But whenever that happens, you do what the Inner Sphere does, and concentrate fire to kill stuff. Likewise, the clan OpFor should not be able to concentrate fire (other than the support forces), until the turn
after the IS breaks Zell. Fire is simultaneous, which would generally allow the IS to get the jump on 'em when they first break it.
As a test, you could offer to play it again with you running the PC's and them running the OpFor. If they can beat you soundly with the OpFor, it might be reasonable for you to agree that the exercise was in fact an un-winnable Kobayashi Maru virtual reality training session, and they couldn't tell the difference due to the drugs that were administered to them at their last meal.
On the other hand, if you win, while playing their PC's, you can roll with your evilest GM laugh and explain how their failure was the crack that caused the entire failure of Operation BirdDog and changed the timeline.