The problem with the Wars of Beleriand is the Doom of Mandos. Mandos had foretold that Noldor will be defeated and that Morgoth can only be overthrown by the aid of the Valar. For the sake of the game we could forget about the Doom of Mandos and make Melkor (as 100 hp general with Balrog bodyguards and zero movement points) beatable. The alternative is to have a time limit. If the player survives the year 545 (that is when Dagor Dagorath began) then the campaign is won. I personally prefer the first option because this is after all just a game and most people do not want to play a game that cannot be won. And beating a 100 hp general is hard enough imo, especially since the Elves did not have siege equipment.