Deliberately unbalanced narrative games

[Valkyrion]

We're wanting to run a storm the castle type game, where there is no complete* line of sight blocking terrain in the attackers deployment zone or no man's land, but there are plenty of ruins and buildings in the defenders zone.

The victory condition is simply annihilation and the game lasts as long as it takes.

No Primarchs, no vehicles bigger than Spartans, no flyers, no deep strike/outflank/infiltrate. 

*there'll be craters and barricades but no structures or ruins

 

There obviously needs to a points difference but I can't really figure out what the correct value of terrain/no terrain is.

We were thinking 3000 points vs 2000 points, but I'm looking for ideas so that we don't have a waste of time complete mismatch. 

[grailkeeper]

Think the classic praetorian battle report in white dwarf was run similarly. The Defenders won if they survived a certain no of turns.

[Brother Kraskor]

You could always run 2k v 3k but if it looks like it's not going as anticipated throw in some casualties as reinforcements?

[Xenith]

Old meatgrinder mission from 3rd ed. Attacker and defencer have the same point total, attacker troops choices respawn whe they die. If a single defender is alive after 6 turns, they win.