We will have to make a distinction between the regular and the warped jellies here because the strategy you need to use is going to be quite different.  Regular jellies can be killed with magic. However, you can not easily kill multiple at a time in the Fremmenik dungeon. Doing it with regular magic is going to be quite a difficult and boring task, and I would not recommend it. If you are going to kill warped jellies with magic, it is going to be a way more fun experience, as long as you do it right and use multiple attack spells. These are spells like ice burst and ice barrage; they can attack multiple monsters at once. You can fight multiple at a time using this method. You can only find the warped jellies in the catacombs of Kourend, so you might have trouble finding a free world. Using magic is going to be pretty expensive. You will have to spend a lot of money on runes, and even more so if you have to use expensive spells like ice barrage. The exp an hour is going to be great, but very expensive this way.