The Magic Heavy Tank is a well oiled machine. It has four separate pieces, and they're all doing something different on their turns. Before you come up with a strategy, let's take a look at how they all work together. You'll notice two bars in the upper left corner of the screen: one for the Tank and one for the Cannon. Now each turn the Magic Heavy Tank will use Tank Hit to damage one party member and lower your GC, while the Cannon uses Volcannon to damage all of your party members a bit. These attacks are both pretty weak—nothing to be worried about. Each turn the Tank Unit and the Cannon Unit will either use Cannon Charge or Tank Charge to fill their respective bars a little, or they'll use Securing Energy to fill their bars a bit while stealing some of your MP. Now what happens when the bars are filled, is that the Cannon will be able to use Full Charge Cannon and do massive damage to all your party members: a huge no-no. The Magic Heavy Tank will gain the ability to use Full Charge Tank Hit when its bar is filled. It will do big damage as well, but only to one party member.
As you can see the Cannon is the biggest threat. However, because of its HP you'll want to target the Cannon Unit rather than the cannon. Once you can knock that out, the tank will lose the Cannon's charge bar, and thus, its most effective attack: Full Charge Cannon. As for how to attack, have Mack spend his first turns casting Powera on Seth and Kaim, and then Minda on Jansen and Cooke. Then have Seth and Kaim use Power Attack on the Cannon Unit while Jansen and Cooke both keep casting Force. Once the Cannon Unit goes down, switch to the Tank Unit and use the same tactics—by this point if Kaim's MP runs out, just switch to regular attacks. The big threat's gone, so you can relax a little. Once the Tank Unit's gone, you'll only be facing Tank Hit and Volcannon each turn. In this time, take a turn to heal up, then go back to using Force and attacking: your third target is the Cannon. Once you take it out, the only attack that the tank will have left is Tank Hit—its weakest ability. At that point you pretty much have the battle won. Keep damaging it until you see a cutscene signifying the end of the battle.
When you flee from the Heavy Numara Corps, you will eventually be intercepted by the General's Attack Tank after leaving the facade. This tank has three parts, the Cannon, the Tank chassis itself and the Magical Engine. It has two power bars, one of the Tank ram attack and the other for the Cannon. The Cannon power bar will be the one to look out for, because a fully charged Volcannon can do a lot of damage to the entire party in one hit. The tank will also sap MP (like a vampiric effect) from Jansen and Cooke whenever it discharges all of it's energy reserves.
Though you can target all three right at the start of this boss battle, first concentrate all your firepower on the Cannon. This is because the Tank chassis ram will most likely target Kaim only (who should have 700+ HP). Kaim should be able to absorb these hits with ease. The Cannon however, will start bombing your entire party for around 100 or so damage. Which is bad news for your weaker members like Cooke and Jansen.
I would recommend using Mack to buff first Jansen's magic attack using Minda, then proceeding to use him to buff Kaim and Seth's physical damage attacks using Powera. The rest of the battle should see Mack used for maintaining the party's GC so as to protect your Zephyr + Flare/whatever element attack combo on the back row. If you do the first phase right, you should experience no more than one charged-up Volcannon in the entire encounter. After withstanding this, dedicate a turn to heal up the party. Because by this stage, the cannon should be close to dead.
Once the cannon is down, treat it like a normal creep encounter. Keep Cooke on Zephyr, Kaim and Seth should divert all their power to the Tank Chassis component, and Jansen will just keep up with his magical attacks on the Chassis as well. Once that second component goes down, rinse and repeat with the Magic Tank Engine. This fight is kind of long, so don't forget to throw Cooke and Jansen a Mana Herb here and there to keep their reserves up for the remainder of the fight. Please note that even though you have already taken down the Tank Chassis, the tank can and will still ram Kaim for the remainder of the fight.
Once the Magic Tank Engine goes down, you win the fight.