Outside battle, you should have S3 grand thaumaturgs. Give them an astral cap to S4 and penetration boosters if you can, and start mind hunting raiding thugs. Even sneaking enemies are hit by mind hunt. Just make sure he doesn't have an astral mage on the province you target.
In battle, enough mages spamming long-range attacks like mind burn, soul slay or paralyze, some will stick whilst he pounds your chaff, and should hopefully preferentially target high HP commanders. If you have a few spare mages to summon more chaff, it will help keep them busy. They will fatigue out eventually - thunderstrike is exhausting - and become vulnerable. Communions can be your friend here. Soulless are also better chaff. The wider AoE of the thunderstrike will probably kill low-HP longdead, so he'll need to fire a lot more if he's hitting 16-HP soulless.
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Here's an interesting little option I haven't tried but might be worth a laugh, using holy magic. Check out Baalz's communion guide (
http://forum.shrapnelgames.com/showthread.php?t=37499), in particular how "Reverse Communion" works. Set one up - communioning increases holy magic as well.
S1D1H2 Thaumaturgs will do, no harm in the odd Grand Thaumaturg as well. If you can get to H5, (Power of the Spheres from the master should help) you should get a penetration bonus on H3 spells. You then have two options. Firstly, Smite. This is a bit like Mind Burn, but probably even better because it will likely kill the Sidhe lord in one hit. The second devious ploy would be Apostacy, a unique H3 spell for Ermor. Apostacy converts an enemy sacred (e.g. Sidhe Lords) to your side. Whilst it's an easy resist (MR+4), if it works, you'll get that Sidhe Lord and all he's carrying. Imagine 10 H5 priests or more spamming apostacy every turn: at least one if not more Sidhe Lord should fail his MR test. That Sidhe Lord will start fighting on your side immediately, and hopefully will survive the battle and permanently be yours.