Get all your characters to level 21 or 22. Upgrade all you can before you start the fight. Your front lines should consist of the Tristan (Gundam), Hal (Gelgoog Special), and Tremmie (Keampher). The shotgun should be equipped to the Keampher because of the speed of the MS.
Have the Xamel Cannon equipped to the Guncannon MS and keep your Gyan Guncannon and whatever mech Gavenger is using in the back lines to charge. When you see the Big Zam about to use his boost attack, switch whichever character has the lowest energy to the ones with the highest. Use Gavenger's Chaff Field. This will block every attack from the Big Zam except his earthquake attack. However, remember the Chaff Field only lasts for a single turn and the Big Zam uses boost attacks for three or four turns in a row. Keep Gavenger's energy up. He only needs to use Chaff Field for the first two attacks because the third is usually earthquake (unless he uses the full charge technique).
Then, Chaff Field the first two and ignore the last two attacks. While Gavenger has the Chaff Field up use your other character's boost attacks. Try to stick to melee attacks and do not use the Guncannon's Mega Fire. Use the Xamel Cannon, because like the shotgun it reduces target armor by 30% and you cannot use it with Megafire. You are not going to win this fight by sheer force. Have Tristan use the Heal All technique and you should have a few repair units in your items screen. Do not be afraid to use them. Also use Offense Hack to keep him from using his big attack. Note: If he uses his big attack without you using the Chaff Field, at least one of your characters will die.