Necromancer tactics against Conflux.

I still don't understand how you plan to defeat two hundred griffons with just 12 vampires.

Usually, by turn 1-2-2, Galtran is already decent. Something like 5-6-3-2. With Expert Armored, he can handle it. The Shield spell is in the spellbook. You need to take 12 vampires and then, with the "Wait" ability, position them in a corner and fly around, attacking full stacks while using incomplete stacks as shields. This way, when the shields fall, the full stacks won't attack the vampires directly but will have already been weakened.
Of course, there's no talk of mass slaughter here. This method has been dubbed "Necro-kiting" in the Heroes community and has been used in random maps for many years. And as for the "cans" (referring to enemy units), there are dozens of them on the Heroes of Might and Magic map.
Using necro-kiting, you can defeat almost any guard force that vampires can drain. I disagree with those who say that Necropolis doesn't have an advantage here (for example, against Orcs, Elves, etc.). Because of the small size of our army, they will be divided into 7 stacks, so the vampires can hold out.

But necro-kiting isn't always necessary. 200+ upgraded skeletons, along with Galtran, can easily defeat a maximum-level "can" on a 1-2-1 map with minimal losses.
In many SOD (Shadow of Death) rulesets, upgrading vampires is prohibited, so skeletons also perform well there.

As for tactics against merging... I seriously doubt that tactics should be tailored to the opponent. The map and your plan are what matter, and the opponent's starting Castle shouldn't drastically change your tactics (the opponent might move, gather a different army, etc.)