Tactics for Necropolis.

Could you play Tower, with Expert difficulty, Slowdown spell, and a few Gremlins, Titans, and Mages? And the Force Field spell might also be useful.

No, it will be difficult to win with Tower. By the time you get Titans, the main Necromancer hero will likely already have Mass Slow, Mass Haste, and Raise Dead, and most importantly, legions (thousands) of skeletons that can take down any Titans and even the upgraded Vampires. Liches will also be shooting at everyone who is protecting the Mages, Titans, and Gremlins.
I think that in a no-rules game, you can only beat Necromancer in three ways:
1) Kill him early, before he can raise a lot of skeletons (with any castle, using archers - Castle, Rampart, Tower).

2) Play much better than your opponent (if there is a significant skill difference).

3) Or, the most reliable method for those who are not pros at Multiplayer and play the same map for many weeks - choose a hero with Diplomacy at the start (I described them, there are only 3 castles) and simply build a huge army to counter the skeletons and go destroy them.