Not that tough to do really as long your character is level 7 or over. The main factors seem to be number of people killed vs time taken vs combo count.
The three toughest levels seem to be the ue-war forest for Inphhy, varrgard castle for Asphar and the phylota flatlands for Dwingvaart and all three are quite achieveable at A grade if you can hit a 1,500 + combo.
For Inphhy, just after you have warped to take on the Pwuck king, equip your level 1 blade and wear the cursed choker (-50% attack power). This is about the only part of the level that there are enough baddies to rack up a monster combo.
For Asphar, the same principle applies but head out to the top of the map through the narrow passage, when you get nearly half-way along there are some powerful baddies which should allow you to get to a 1,500 combo.
For Dwingvaart, concentrate of retaining as much health as possible until the end of the level. When Asphar and Inphhy appear they bring a massive army with them, equip the level 1 blade and the cursed choker a 2,000+ combo is easily achieved.
Just remember to switch back to your power weapons and items once you've reached your combo!