The Battle
Vortex Lvl 2.
Seriously, that's it! That's all you need for this fight on any mode! :p
You don't have Vortex Lvl 2... >_>
Well, use any DTed attacks then :p
Boss moves? Look, Mundus 3 doesn't have any moves - oh all right, then. At least twice in the battle he will send a green laser, which you can easily evade just by standing right in front of him. Bits also spatter off his decrepit body, which can hit you for minor damage. Other than that, he is harmless - until he reaches the end of the room and crushes you with his ginormous bulk... :p
You've got plenty of time to finish Mundus off before then, though. When the time comes for Dante to use his guns, you don't need to waste time firing at Mundus to restore the DT gauge. There's a glitch whereby you can restore it without any shooting. Hold down R1 + shoot button for about 3 seconds, then let go of the shoot button - your DT will automatically refill. DT, shoot, and... "Jackpot!" :p