AK, if you're DPS, you should grind Ifrit until you have the Ifrit Weapon. I would suggest 2 or 3 pieces of DL. This is because the Demon Wall is a DPS race.

Don't know about the cannons as I play on the PC. But if you can't interact with them, it's probably better to let the group know, a healer or tank can jump on it for you.