Strength potions boost your strength, which in turn helps your melee damage (damage per strength point is 0.4, 0.14 or 0.1 for a warrior, survivor or wizard, respectively.
For secondary stat calculations, see
differences between classes (minor spoilers)
or
Early Starter Tips (NON-Spoiler Version) (contains much of the information in the above topic).
After killing the necromancer, if you run west and a little south, you should be able to get to a relatively open spot by the wall, with only a couple skeletons following you. Once you are not surrounded, you can advance slowly, and take them on in reasonable sized groups.
