If you can go on boss runs, anything you can survive that casts physical immunity is good (no accuracy is required to do 1 damage during immunity, so you can leech at any level). Dunno if that works for Thieves though since their HP is pretty low and it's mostly the strong bosses that have physical immunity.
I'd say it's not worth leeching until it's something your Arch Mage would consider training on alone (possibly Harps or Wolf Spider, though the new Windraiders are good too - esp. on the first cliff level they spawn in huge mobs. Trade-in from the ninjas should be available though, no point collecting badges otherwise) I think you can dark sight it up to the cliff...
I'd say it's not worth leeching until it's something your Arch Mage would consider training on alone (possibly Harps or Wolf Spider, though the new Windraiders are good too - esp. on the first cliff level they spawn in huge mobs. Trade-in from the ninjas should be available though, no point collecting badges otherwise) I think you can dark sight it up to the cliff...