A large number of low-level monsters are not relevant here, of course: in 2-3 weeks, there won't be enough of them for a rush (there's not enough dark energy)...
Let's remember that when leveling up a Necromancer hero, the "spell power" stat grows predominantly. Therefore, one of the ways to implement a Necromancer rush (and, in essence, the most effective and fastest) is with Fire Warriors. At level 15, the Necros hero will summon, taking into account the "Master Summoner" skill, about 40-50 Fire Elementals; adding the "Ring of the Elements" to that, we get almost 80-90! The second option, in terms of difficulty, is to focus on Chaos. I won't talk about the corresponding build for this, as it's obvious. In principle, with strong neutral creatures, this may be possible no later than the 4th week; of course, you need to take into account the specifics of the map we are playing on and the availability of the corresponding spells...
It is unlikely that an Orc will be ready for such a scenario at his 15th level: the probability that the Orc will get anti-chaos or anti-summoning is 5%; the centaurs will not deal significant damage to our units, and the Fire Warriors will destroy everyone while our "meat" holds out.
Regarding Kaspar. Similar to Daleb, this hero is ideally suited for secondary roles: clearing medium-difficulty neutral creatures, capturing mines and artifacts (not relics, of course), "gathering," etc. I'll explain why. For example, let's take Kaspar at level 15, assuming he has maxed out the "Mechanics" skill. And what do we have:
- a catapult has 400 health points, which can be destroyed in 2/maximum 3 shots or magical attacks;
- the catapult will heal/revive/kill, taking into account Kaspar's ability, at most 175-200 health points (this does not take into account the hero's stats and the parameters of the unit being attacked by the catapult).
So, who can you rush with such a "miracle" hero?! Well, maybe only a "noob" player on a map they don't know well, or, at worst, on the "Outlands" map...