I'm not that great at the game, but I think the replies you'll get will be few and far between so I'll give it a try. At the point you are at in the game, I remember asking myself this very question and the place I spent most of my time leveling is the swamp (the green area that's not quite as dark as a forest where you move very slowly) which is south of Ruto town. In the swamp there are a couple octoroks each giving 10 exp (i think) which is much better than battling slimes and they're not too difficult either. After a while you'll notice that they attack in the exact same pattern nearly every time so they'll become pushovers. You always want to battle the smaller enemy silhouette on the overworld screen in the swamp because the bigger enemy silhouette just adds annoying birds.
Unless you have amazing Zelda 2 fighting skills, you almost have to level-grind before attempting Death Mountain.
If you're looking for more experience more quickly, I suggest going through the 2nd Palace to fight stronger enemies but not defeating the boss. I don't know how familiar you are with Zelda 2, but the biggest tip I can give you is to be patient at the beginning of the game and do most of your level-grinding then. It is very easy to manipulate the leveling system to your advantage by not defeating the bosses until you are at a higher level. This is because defeating the bosses automatically brings you up to the next level. So doing this allows you to level up your last 6 steps (for each of the first 6 bosses) without battling enemies over and over again. I didn't yet mention that this method saves you a ton of time because the last couple of levels you gain require very high amounts of experience. Although to be fair, the enemies later in the game do give a lot more experience so the advantage of this method is not nearly as great as it seems at first glance, but it still saves you time in the long run.
