(( Hey Gigataur,
To semi-quote Engineer from Team Fortress 2: "How do I keep a mean miscreant from creating in my posterior a superfluous hole? Gun. And if that don't work... more Gun."
A couple of thoughts.
1) The Necromancer is worth targeting, and should be a priority. Most of his summon and re-summon power will be from the Necromancer. Not to mention that I think the Lore of Vampires lets Vampires regenerate wounds with successful spells or something. Removing the Necromancer will also cause the 1-turn instability test... which does have the chance to grant you free wounds on everything.
1-A) Anyway, most Necromancers tend to be Level 4 wizards that usually have that ability that lets them grow units past their original size, while Vampires tend to sit at 1 - 2, depending on cost. Taking out the Level 4 will greatly empower future magic turns. Which'll greatly increase your chance to cast your Augments and Magic Missiles.
1-B) Don't forget, on low-power Magic turns, you can try to Slim-cast spells. Our group calls "Slim-Casting", when you just use 2d6 to try to cast a spell. With 2d6's biggest average as "7", you can try to cast spells that normally need 11, with a decent chance.
1-B-1) Very poorly calculated statistics gives you about a 50 - 50 chance to cast a Power Level 11 spell on 2d6 + Wizard Level 4. Banishment has about a 60 - 40 chance on 2d6 + Wizard Level 4.
2) In my opinion, it is usually better to eat away at the skeletons, since they tend to be WS 2, T3 / 5+ Armor Save / 6+ Parry (in other words, easier to cause an unsaved wound)... while the B-Lord tends to be redonkulus. If you can crack enough skeletons, you stand a better chance of being able to out-pace the B-Lord, in turns of damage caused each turn. Still, though, it'll take a turn or two to eat through the skeletons... even while they are crumbling. All the while, the B-Lord will be chewing into your unit as well.
2-A) And, getting rid of the Necromancer will make it less likely the skeleton mob will simply hobble back up from the damage you are causing.
2-B) Sometimes it is advisable to risk the Challenge with an under-powered character, on the idea that you're other guys will be crushing enough skeletons to counter the +5 CR that the B-Lord should be generating as a Challenge result from Overkill.
3) Overall, though, what you could do is avoid the B-Lord and his escort. Slow it down with Miasma or Curse of Anheir, or terrain impediments, or push it back with the Wind Blast, so that it can only get to risky-charge range for infantry units.
4) Light usually does above-average against Undead of all types... but it'll still take some time for it to really start chewing up the undead, as long as the Necromancer is around.
4-A) You can estimate Banishment, if successfully cast, will take out some 5 - 10 skeletons a turn... but a good Necromancer can generate twice that many back fairly reliably, as the Necromancer will often try to do that Bubble-Raise-Dead spell thingy, that grants new skeletons/wounds to all sorts of stuff. Hence why taking out the Necromancer should be a priority.
4-A-1) Remember, though, you'll have to bunch up your Light Wizards, as the Power-up effect of Banishment has a 12" range.
4-B) Overall, though, the big power-up of the Banishment doesn't come from the High Strength, but from the volume of hits that it can generate. 5 - 10 skeletons... easily repairable. But, if you're doing some 10 - 20 skeletons worth of damage a turn, that'll start to take the steam out of the B-Lord's escort.
5) While not necessarily as specific as Light, Fire does pretty well against Undead. Fireball is a solid, basic spell which is cheap to cast, and Fulminating Flame Cage can often "Freeze" a unit in place for one turn... as, if it moves, all models suffer a hit.
5-A) Piercing Bolts also penalizes Steadfast-sized units. Against the 5x6 unit-size you were talking about, Piercing would be causing 6d3 hits (average around 12 hits).
5-B) Fire also counters Regeneration, which Undead tend to have via skills, spells, or Corpse Cart effects.
5-C) Also, if you concentrate your Fire (pun intended), subsequent Fire spells get more powerful, without having to use additional dice, via the Lore Attribute.
5-C-1) So, you singe him with a Fireball, and then trap him with the Fulminating Flame Cage.
6) I would say, with your two tanks, you could probably drop one cannon, to free up some points for some other stuff.
6-A) Be advised that, even with wounding on 6+, a B-Lord can crack a tank-shell, due to the sheer number of attacks / re-rolls they tend to get.
7) Dwellers is an expensive spell to cast. At a minimum, you need 3d6 to attempt... to reliably cast it, you need 5d6. Given you said you were having slim-magic phases (4 - 5 Power Dice), that pretty much eliminates it from your casting selection. As... if you try to cast it, you're opponent is just going to throw all their dispel dice against it, since you pretty much won't have a whole lot of Power Dice left to try to cast something else.
Anyway, just some quick thoughts. ))
