You don't need to level to 25 for the end game when playing on easy. There's only one difficult fight in the end game, and that's the first one, so you'll immediately know if you can finish. You may want to bring some bombs though, just in case.
To level up in Easy, doing the catacombs is the fastest way I think (has been a while since I did a playthrough). Anyway I think that finishing the catacombs you should be around level 20 at least, enough to finish the game at that difficulty level!