True enough. Just finished a Scryer Vortex campaign where Rictus went balls deep on the DE's and had most of Naggaroth conquered by the time I consolidated Lustria. Was quite a the suprise compared to how he normally does in Mortal Empires.
For your current situation, you could just turtle and wait out the growth for the tree and then just do the final battle. If you want to go on the offensive, I'd recommend going with a Treeman stack and just loot/sack your way to their Tier 5 settlements and blow them up. They can ruin dwell but it will cut their recruitment down to just skavenslave and clanrat trash, which is much easier to handle than weapons team stacks.
Treeman stack wouldn't be vulnerable to ambushes and you can have some waystalkers or glade captains to scout ahead and also scout around your Hearthland so you can see their armies coming. You could also occasionally capture a settlement if you need the replenishment and then just abandon it or setup an ambush to bait the armies nearby to you, since the garrison will be damaged and weak.
Keep the Sisters back to cover defense and you could even set a Ambush bait trap with a level 1 lord sitting around to bait them in with the Sisters in ambush next to it.
Also, if you can, try and have a port and send out a hero to try and get some trade going with some of the other factions. If you can get some decent relations, you may even be able to get them to join in your war, since some of the Order factions like HE and Lizardmen won't like a large skaven faction like that.
