And how does he differ from other Necromancers (is he just a skeleton specialist, with a slightly stronger army)?
At the beginning of the game, there are few of them (and the bonus is small too).
But if you play for 2-3 months, you can find something to counter them.
Of course, it's better to fight them with ranged units, although Swamp and Inferno can defeat them without ranged units (ranged units are necessary for convenient development, otherwise they will be overwhelmed in the second or first turn, and there will be no point in them).
The thing is: there are rarely armies of equal strength in the final battle, so... either you defeat Galtran... or he defeats you...