Only high levels in ftp are greaters, and revenants. anyways i don't remember when i did it but it wasn't to hard of a deal, alot kill at lower levels, and protect prayers arn't needed, i don't believe elvarg deals magic/range damage if u have the level protect from melee would be best.
As for being able to go back and kill Elvarg no u cannot.
And a tip is open the secret door out of the dungeon first in case u die it will be easier to go thru that door instead of going on the ship again.