You unlock Legendary difficulty after beating the game on Heroic. Legendary is not much harder than Heroic, the enemies have much more health and if you go down you die faster (less time for revival).
You have lots of skills unlocked by the time you play on Legendary so you should be able to rush through the game without too much trouble.
I found that the scout was not too helpful for this playthrough since he could not deal so much damage. The mage has very powerful ranged (special) attacks. Her energy ball did 30-50000 damage by the end of the game.
I suggest - as on Heroic - playing in koop - one player deals damage and the other supports and revives if necessary. For revival - the dwarf can learn a war cry which revives instantly all downed characters. And the scout can make himself invisible - once he is the enemies will not attack him and he can revive other characters unchallenged.
I think I got through the game on legendary with a friend with mage/scout combination in about 5 hours, rushing.
The only hard areas are the areas where you have to protect someone or something. In chapter one summon your bird for the troll at the gate. In the chapter where you have to protect the dwarfs use the war machines (for some reason I could NOT use them on legenary, my friend could surprisingly) and when you have to protect the city gates and all these flame monsters come you "simply" have to get them all. This takes some practice - again - this is easier in coop when you do not have to worry about your KI colleagues dying all the time.
Good luck. In the end I found Legendary the "easiest" difficulty level. I even reached the level cap of level 40 by the end.
CrippleThreat8 added that in coop this achievement seems to work only properly if all players have beaten ALL previous levels already. It seems to corrupt the save file if a player has not beaten the entire game on Legendary when he finishes it in co-op. So if you plan to try anyway you might want to create a backup-save on an USB stick (if this game supports this).