In Dragon Quest Heroes: Rocket slime (DS) I found the ultimate tank battle against your best friend to be the hardest battle. First off, you don't have any of your crew to help load ammo and wreak havoc on his tank, second, his tank has almost double your tank's maximum upgrade, third, he launches ammo at you like crazy, and fourth, once your tank is down to 0, he runs as fast as he can to destroy your tank's core and you have to kill him or else you're fucked.
I had to let him take my tank to 0 and then kill him twice just so I could get enough time to bring his tank down and make a hyper mad dash to it and destroy it's heart, still takes me a damn hour. (Most tank battles take a little less than 10 minutes, still pretty long)