Did you check the miller? He sells potions, though as Niner said, if you have the ingredients you can create your own potions in his cellar.

Level 1 of the healing spell comes in handy, and would help you to conserve potions.

For a melee warrior, level 1+ of Life Leech helps with healing, and Bleed (up to level 10) helps end fights sooner.

Do you have a bow? If you keep withdrawing rather than let yourself get surrounded, the bandits should spread out a little bit as they come after you (especially if some of them concentrate on your creature or a summon). You can then do some damage to them as they approach, killing or softening them up for a melee attack when they get close enough (if you are playing a warrior, Rush Attack rangers or mages if you can not do damage to them at a distance faster than they can to you).

If you visit the enchanter, do you have a high enough level healing aura to justify the malachite gems required to add a couple enchantments to some jewelry? If not, maybe a few Startic Charge auras would help do damage a little faster. If you have low level armour enchantments, maybe replace them with Life Line, or something.

Each time you level merchant inventories will be reset (until you next get close enough and run the crosshairs over them). If you can not take out a few bandits at a time going up the path to the battle tower, you may want to head to the fjords. There are level 18 opponents along a winding mountain trail (I was level 16 after the first encounter there, and made it to level 17 at the bottom, and crossing over to another couple beaches made it to level 18, though the last 4k experience for that was tough to get, with only level 21 imps left).