Spells are a pretty bad way to try to counter this old lad- if he isnt paired with banner of the world dragon its likely he'll be with caradryan and so will have MR3 (unlikely not to have any MR) so wil have a save against spells like the golden hounds. Characteristic tests or straight dead spells like swellers and final transmulation are great... but a little hard to pull off against Teclis without irrisistable force: he comes with his own scroll which is almost certain to be used if you are targeting a spell at him that can kill him in 1 go (and may well stop you using that spell again) and if not then he still has +D3 dispel dice and +5 to dispel... not easy.
If I was going to use magic then I would hope to have something like a lv4 with life, a lv2 with life and a power scroll. Lv4 bufs your army and rolls spells first (does not take dwellers), Lv2 almost has to get dwellers (certainly will if the Lv4 didnt roll it and didnt change anything to earthblood)... that lets you march the Lv2 away from all units turn 1 and then throw <6 dice at dwellers to force it through.... without a banner of teh world dragon or some serious luck Teclis would die. Its also not a bad set up against other armies... but it is broken and beardy as all hell (BoLS made a list top6 broken things in 8th- Teclis, power scroll and dwellers all made it on).
If you have any directed (sniper) shooting then Teclis is in trouble. Things like the hunter's bow, pageant of shrikes or hotchland longrifle are all pretty sweet against Teclis... but you'll often find he takes life and so that is pretty irritating too, as you mihgt put 2W on Teclis, but then by casting a few spells (and you wont stop him casting at least 2 a turn) he'll be back to full wounds... arrgghhh
The best way to kill him is always to get into combat and munch him... but you come into problems here too: the standard suicide assault squad that I would use to kill enemy wizards wouldnt work on Teclis. First he's a lord and so has 3W meaning that you need to have 3A to have a chance at him, and ~6 to make it really likely but the bigger issue is that he will normally sit in a bit unit of HE, with huge numbers of attacks, rerolls to hit and they'll go first... Dum dum derrrr.
However, on the up side you dont need all that many to survive and you can jump all over a massively expensive character. So sending a unit of 30 clanrats into a fight they cannot possibly win is fine: you might lose 15 rats and have no chance of winning, but you can kill Teclis (and mebbe a few bonus HE) and draw the enemy out of position if they pursue you while only risking a relatively cheap unit. Almost all armies have something they can try this with... about the worst example would be ogres, but even then they can use gnoblars for this purpose (if you ever manage to get them into position).