By the current rules, a character with thaumatergy could do just about anything as long as they had a decent lore and enough time.
Actually, that's not true. Several things limit the power of thumaturgy;
1) Thaumaturgy spells are fairly costly. To reliably kill someone, you need 30 shifts of power. Trapping the Erlking should be comparable, if not more difficult. And that is for fairly small, personal effects. Meteor from the Sky would be at least 60 shifts, if not more. So Lore of +5 is not going to cut it.
2) Skill declarations are not guaranteed and they should rise in difficulty as a skill is used more or the spell is getting bigger. Magic circle for a 10-shift ritual may be some chalk line on the floor. Magic circle for trapping a major entity is so complicated that only grandmasters could make it.
3) You need to control the power. A discipline of at least +5 is required to safely control power and interruptions (reflected as compels or aspects) can worsen your roll and make you fail - that makes the power explode in your face.
4) Thaumaturgy takes time. If the preparation is just a few quick declarations, nothing bad happens. But if you need to be focusing for hours at a time and then speak the incantation perfectly for an hour-long casting time, Endurance is going to limit your skills. This means you won't be getting as high rolls out of them - which leads to failures.
5) It bears mentioning twice; losing control of the power is BAD. If a ritual would take more than a scene, events keep unfolding in the world and may affect the ritual.
6) Thaumaturgy needs a sympathetic link. Getting someone's nail clippings, blood, True Name and the like can be an adventure unto itself.