I'll occasionally go down to 2 Dissolves because my opponents rarely play equipment that I feel compelled to destroy, but there are pieces of equipment that I always want to destroy when I see them, so it's safer to run 3+ Dissolves.
As for dealing with an armored Necromancer, either choose to destroy all of the equipment giving Flame -X and start landing fire attacks, or remove the Veteran's Belts and just power through the armor. Without Veteran's Belt, no matter how much armor they have, you can still get damage through by throwing enough dice at the problem.
Bleed + Deathlock is another reasonable solution, particularly for a Beastmaster with 4 Dire Wolves.
As is stacking Corrode to strip the Necromancer of Armor and destroying the Wand of Healing.
Honestly it sounds like this Necromancer's clock is so slow that I don't see what the issue is. You should have plenty of time to adapt.
If you're struggling, just play a Gate of Voltari Wizard with 4 Teleports, 4 Devouring Jellies, and a Darkfenne Hydra or 2. Play all of your Jellies, then lock the Necromancer down with Force Crush in a zone with 4 Jellies. The Necromancer won't be able to survive that for long. Once he runs out of armor, your creatures should be able to kill him in 1-2 rounds of concerted attacks. Besides, Watergate Wizards run 6 Dissolves and 6 Dispels anyway... If you're concerned, run 6 Acid Balls as well. I don't see how this Necromancer build would even have a chance against a decent Watergate build. There are things that every mage can do against an armored Necromancer, but the Watergate Wizard is the build that should hard counter it.