Once you get the Eldritch upgrade you will notice a big swing in the dungeons, especially with the sniper upgrade.I would start by purchasing Event treachery after your upgrade in order to add some punch to your Lt encounters.
I recently won a campaign using Shackles (only got the benefit of the starting card, never played the potions/market one and the heroes gave up before Silver) and never missed a raze roll as a result. It was pretty sweet.
If you manage to raze Dawnsmoor that is awesome, will increase you CT on a weekly basis which I found useful for other Avatar Upgrades. Do they have the boat yet or are you safe there for the time being?
If you can get them to chase your Lts around it wastes time for them and adds to your CT count. The Transport Gem and moving your Lt up to Frostgate can accomplish that. Won't work if the heroes are going to ignore the siege, but talk up the skills you are going to make infinitely harder to get and watch as they struggle with the decisions. Frostgate has Divine Retribution for example, if Zyla were to get it she would be the worst target ever, 1 CT and lose all the nearby creatures if you aren't careful.
Hope that helps you out a little.
They already have a boat, just bought it. i have 2 siege tokens on dawnsmoor not sure if they are going to chase Alric. Thanks for the idea with the transport gem to frostgate. Not that i am very afraid of Zyla with retrebution - she can only be killed from a distance and with snipers it's not a problem to be far far away from her when i kill her. But sieging frostmoore is nice as the heroes needs a lot of turns to get to it.