Immediately Lock-On to it at the start of the battle and use a Fire Spell. This should deal a great amount of damage. Attack with your Keyblade a few times, and the Heartless Boss should plummet to the ground, stunned for a short time. Attack the beast until it awakens.
Once it recovers, the Avalanche will attempt to stop you with a variety of attacks. First flying a short distance away and then landing with a mighty shockwave, the Heartless will then fly low to the ground in a straight line, dealing impact damage if you fail to dodge. It should be noted that both of these attacks can be nullified through use of the Block Ability. Attack whenever possible, but beware! The Boss will then shoot homing balls of ice at you and Xion. These deal heavy damage, so stay as far away as you can.
Bring as many healing items as possible into this battle. Make use of your Limit Break. Follow this strategy, and the Avalanche should be relatively simple to defeat.
Stats & Abilities