Yeah...
Try taking a Necromancer or a Wizard instead of a Galtaran, or a hero with a specialization in magic (Necromancy)...
Yes, skeletons aren't the main offensive force for Necropolis, but they are a quite powerful supporting force. I don't think there's much point in taking a Galtaran and increasing the stats of skeletons, especially at level 1...
To conquer a Stronghold on a small or medium-sized map, attack them before they build up their creatures, build only troops, and attack on the map as much as possible... A Wizard with vampires will be more fun in this case, as there will be quite a few battles with no losses.