Ok couple of different tactics I'd suggest here: (Sorry that it turned into a bit of a wall of text, I will add in as a disclaimer, that I likewise don't know much about hell cannons, and haven't played vs chaos.)
Mobility - His army isn't going to be very mobile, so separate them so you're only fighting 2/3rds or 1/2 of his army at once.
- Use a Varghulf to march up in between his units, so that they don't have a forward arc to charge you. Then you can pick and choose when/where to rear charge after he's engaged you, and gimping his marching capability in the meantime. He won't chase you, if he does you can easily move out of his frontal arc again. Could also kill Hell Cannons while he's behind their army.
- Use the terrain to your advantage. Block 1x flank with impassable terrain if you can. Even a forest etc so that if he does risk going through it, he'll lose steadfast. If you do this, it cuts up the battlefield, anything from that flank has to go the long way round, and won't be involved in combat til T5 or T6. If you're only fighting half his army at a time, you'll crush him.
Magic: - He doesn't have any casters, so you'll get off most of your spells without too many problems. Particularly since you don't have a coach.
- Use winds of Undeath to your advantage, he doesn't have any magical attacks with just Maurders right? So you can tarpit his units with Spirit Hosts and only have to compete with his +4 static CR. WS4 T3 with not much armour you should be able to only lose combat by 1-2 each round. Thus each casting should keep 1 of his units out of your hair for a turn (Particularly if you can get a flank or rear charge). At worst, park spirit host or zombies 1" infront of his unit, at a wierd angle which means an overrun would be pointless. Then he only gets a 1" charge forward that turn.
- Magic Missile him as much as you can, Remain in play spells will be useless though as he doesn't have to cast, so will have PD to burn in his own magic phase to dispell with.
- Buff up 1 or 2x units of core troops to excessive numbers, and have them take his initial charges. They just need the numbers to sustain the casualties from his charge.
- imo go support Lord instead of butcher lord. You really don't want to risk 12x Str 5 attacks in the first round of combat. I love combat lords, but they really are just too risky, particularly against high str rank & file units. If you're going to dominate the magic phase anyway, might aswell take full advantage of that, so MotBA etc. He'll be -3 or -4 to dispell, so if you 2 dice something, he has to 3 or 4 dice it. With MotBA you'll average 9-10 PD a round.
- imo ditch conrad too, get a vamp with good augument/hex spells. Maybe lore of shadows, although bear in mind that he can easily dispell the remain in play spells from shadows before his combat phase. However not before his movement phase, so you could have your 2nd vamp, on a hellsteed (he can't shoot other than his hellcannons), move up infront of / behind his ranks without risking being charged. This Vamp could also probably magic down his hell cannons? Particularly since he can probably steed of shadows to be within range of them T1
Combat: - Survive the first round with each unit, and with the right buffs, they'll be running the next.
- Let him charge one of your big core units, so that you can rez units + then retaliate with counter charges etc once his unit is exausted and doesn't have the +2 Str bonus from flails.
- As said earlier, ranks of 5 wide for your units, and go as many ranks deep as you can. Apart from the first round of combat, when he destroys you with his flails, combat should be mostly even between your unit and his (you'll usually lose, but you can replenish your models more easily than he can, and might get lucky and win a round) . Most important thing is, you need to survive that first round of combat, with a decent amount of ranks remaining.
- Hold back your GG/knights if you can, then charge them in the turn after he's charged you.
- Your Varghulf in behind his units will allow you to boost up your CR after he charges you. +1 Charge, +2 Rear and should average 4 kills a round (With maybe 1 wound in return max). Only charge after that first round though, otherwise he might kill your Varghulf first round of combat.
- Use Helm of Command, he's WS 4, which is unfortunate since he'll still be hitting on 4+, even if you give it to your Lord, so can give it to any vampire. But 4+ is significantly better than 3+, also then you'll be hitting on 3+ instead of 4+ which will boost your kills too. Use this on whichever of your units is to his front, preventing casualties is the main goal here, particularly to your big unit of skeletons / ghouls, since you need to have more ranks than him.
- If you win a round of combat, he loses frenzy. Then you'll have a significantly better chance of winning subsequent rounds, even if he's still steadfast and doesn't flee.
- He'll likely go 10 wide, so he won't be that deep. If he isn't 10 wide, and so is quite deep, then you'll win combat more easily, you'll just be gridlocked in place for a long time though as it'll take ages to remove steadfast from him. But since you're 5 wide, it hopefully won't be too long until you outrank him.
In general, I think that the main tactics for you, are to divide his army via terrain + using Spirit Host / Zombies and the Varghulf to prevent marches / 1" charges.
Then you dominate the magic phase, whittle down as many of his units as you can from range, buff up your own units
survive his charges, and then butcher him with your own charges. Since he's a bit of a one trick pony really. First round of combat is going to be brutal, but after that it's an almost level playing field (He'll have 2x the hits on you, but you'll rez them back each turn). But when you add in magical buffs / helm of command / rear charges, you should be getting the better of him. It's just that first round which will destroy you.
This tactic does depend heavily on you surviving that first round of combat, with ranks. You'll take some massive casualties during it (Probably 29x WS4, Str 5 attacks vs your unit, without any buffs, you could expect to lose an average of 14 skeletons from his charge, and maybe another 10+ from CR, more if his lord is in the unit or if he gets lucky rolls). With the right buffs though, (particularly if you're use ghouls), you can probably more than halve those casualties.
