I'm only playing on Easy because I want to unlock all missions on all difficulty levels. About Leviathan, concentrate on the organ that steals your Orbs. once it's down and out, concentrate on the main part, and then start dodging the lasers it shoots. (Griffon homage anyone?) Leave the Hells alone, unless they really become a problem. You'll run into some new monsters in the next mission, which are also hard in groups. Yeah, Agni & Rudra were hard, but I managed to finish the remaining twin quite easily. The best tactic would be to attack them equally, then finish of the first one and immediately after, the other. Agni & Rugra is by far the best Swordmaster weapon, although other Devil Arms are good too. Stuff costs an arm and a leg (or two).