Use Snow, Lightning, and Fang and use the following classes for each respective character:
1. Sentinel, Ravager, Commando
2. Sentinel, Commando, Commando
3. Sentinel, Medic, Commando
Optional: Sentinel, Medic/Ravager, Saboteur
Snow should be pretty beefy atm. I did this the other day and he was around 1800 hp. If you want, start the battle off with the optional class so that he has deshell, deprotect, and slow on him. Then for most of the battle, use the first class. You can switch the the second one from time to time to get some nice damage. When he Attacus gets his third sword, Snow starts to take some heavy damage. If he doesn't seem to be in danger, keep with the first class. Otherwise, just stay with the third class until the battle's over (His stagger should be around 400%, so Fang alone should be doing a lot of damage to him on her own). And since Snow has Attacus' attention, you don't have to worry about the doom clock. Shouldn't take too long. Maybe about 7 minutes or so depending on your characters; but since the target time is around 19 minutes, you'll get 5 stars off of him easy. And don't forget to boost your stats with items prior to the battle.
Note: This was the first time I used Snow after being able to customize my team. I just built his Sentinel stats throughout the game until it maxed then added the remaining CP to his commando class.
Coincidentally, I came in here to post that I beat Attacus the other day, but forgot that I had Hope equipped with the growth egg. 75000 easy CP down the drain.