Talk to your trainer and enchanter to upgrade your runners' equipment, and give them a better chance of being successful. Also, once fully upgraded they will get back faster; you just have to wait until they are out of the tower, and then teleport out and back again. If you take the teleporter to the exterior sentinel island teleporter shrine, you can use the dragon stone to teleport to the throne room and leave repeatedly to build up ingredients for whatever the strongest healing potion is that you can make.
If you don't have enough gold to upgrade everyone, start with the gem runner and send him out for diamonds to sell, so you can upgrade the other two (the ore runner can fetch malachite ore, but they are not as valuable as diamonds).
Level 1 of the healing spell is handy, as is Life Leech for a melee character. There is a Potion Efficiency skill that can also help conserve potions, but potion ingredients are easy to get once your runners are upgraded.
If you take the path past the depleted ore mine you can find a few more merchants; when the path splits stay left and go up to High Hall (need to clear an area of trolls before the merchant is available), or right and go down to the harbour. Some merchants can have potion formula and/or ingredients.
On that path you can meet a traveling merchant in a one time encounter; hit the trade button to get a malachite gem, which is not available if you start trading from the dialog option.
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