Yesterday, me and my brother where playing some matches and he did this tactic to me:

1- He used the action of lobber crew to kill me one unit, then, with countess iseara he used necromancy and put back the lobber crew to kill me another unit, and then he did the same thing 3 times as he had over 10 resources.

Is this tactic valid? As a dwarf, what can I do against it?

As long as you have the resources, you can use that combo as much as you like. Not sure what the Numberless Graves has to do with the situation though, it doesn't key off sacrifices. As to what to do, have Dwarf Ranger in your QZ, preferrably 2-3, sac some other Dwarf unit and ping Iseara for 3 dmg, it'll kill her eventually. Or just wait until you have a ton of units in your discard pile, play Reclaim the Fallen and make on monster attack. Repeat to burn the other zone.

Tanks!

Oops, the numberless graves doesn't do anything in here