You can complete the battle against the troll without taking any damage; you might need those 5 mana later. Also, there's an interesting feature that I already mentioned: in certain areas, you can dig a hole, and this hole will act as an obstacle, preventing passage. In Mr. Fat's zone, you can use this to block the garrison, preventing the companion from reaching you (or delaying that moment). I tested this in version 23.5 (although the portal locations in Mr. Fat's zone are slightly different there), but it should also work in the latest version of the map. Whether or not you should do this is another question...