Isra and Vedomina are not bad alternatives to Gosha, but they are quite limited by the rules.
As for how to kill him, the best approach is to rush him in the first or second week, before he gets too strong. The longer you wait, the harder it becomes because the skeletons multiply at an alarming rate – necromancy plus the converter (and God forbid, peasant ambushes).

The converter is forbidden to build on both the Portal and HotA maps, and Isra and Vedomina are weak heroes because they only start producing skeletons at a rate comparable to any other necromancer with expert necromancy after level 10. However, in the first 1-2 weeks, the battle is against numerous but weak units, and you need to immediately produce 30% skeletons, not 21%. Considering the strategy of playing against Necropolis (rush), specializing in Isra or Vedomina simply won't pay off; it's better to use Clavius or Tamika.
One peasant ambush is far from ideal; it's much more useful to have 2-3 secondary dwellings and a few level 1 ambush buildings from the first week, plus heroes in the tavern with either gremlins or troglodytes. Specialists of rank 1 with 3 stacks are also worth their weight in gold (Shakti, Grechin, Ignatius, Dragon).
With half of the described advantages, the production rate is at least 250 skeletons per week.
On HotA, if you can get Galtran at the start, the prospects are always good, but with the Portal rules, the Necropolis is less stable.