The trick to this fight is the field element. I suggest having Emil equip Tenebrae and spam Demon Fang until the main field element becomes dark.
Doing this will make Decus lose his regeneration ability.
Also set your monster strategy to Magic, so whenever Alice tries to heal, your monsters will banzai her and interrupt her spell.
As for Decus, never underestimate the power of Freerun, also try to have Emil hit Decus with an upward slash to knock him into the air. Your Emil should have Sky combo 2 so you can hit a 5 hit aerial combo on him in the air and if you have EX attack 2, you can charge the last hit in the combo to knock him down to the ground so you don't have to worry about him hitting you as you land.
Hope this helps.