all of them are good to various degree but likewise none of them are really necessary compared to game of winter in 2. Don't really bother much if it's too tedious, you won't lose anything of value.

Thanks

Don't play any map as an attrition war. That flies in the campaign only because most of those missions don't have a fail state.

The DLCs reward you for being proactive. You want to hit the overworld as soon as possible, kill heroes in your dungeon to make use of the prison, and get at least one titan unit.

Trust me, it's not meant to be an attrition war, you are supposed to defeat them before you are overrun. Because being overrun entails losing, and losing is directly in counter to the point of the game: winning

I was not trying to play as an attrition war, I was trying to clear an over world full of high level enemies and guard towers while waves of hero's spawned every 60 seconds and it became an attrition war. Even after killing the outpost next to the teleporter the enemy waves didn't slow down.

It happens. Some of the missions are tougher than others. I guarantee you that if you try it again but start harvesting evilness faster, maybe try a different upgrade path, you can do it, but the timing will be close on some missions, it's just the way it is.

I've personally found that upgrading Thalya for a lot of these missions is hugely beneficial compared to the base game, for example. Not only does the fireball start killing enemies rather than chunking them, but those little shadows gumming up the melee helps your guys survive longer. I'm always getting 5 or more succubus units and upgrading them to limit incoming damage as well.

The first few DLCs are harder missions than the ones in the main campaign, but nothing too different. Clash of Gods and onwards adds some new spells and traps which I never found that useful but also changes how some of the enemies work. It adds a day-night cycle in which heroes only attack the dungeon during the day and may choose too set up a camp underground instead of attacking the dungeon heart. Spiders will also occasionally expand their nest through the underground and make new spawners. Clash of Gods also has more missions than the other packs, which typically only have 3.