People talk about how Lucanis isn't actually the one who's impressive and say he's only able to get a shot a Ghilanain because of Spite... But his handling of the Spite situation is deeply impressive to me.
He doesn't just hold out for an entire year, he makes a deal that means his worst concern is shared custody of his body. Most mages can't even do that.
Calivan says, "It was supposed to take a week, at most!" Thedas has five day weeks (we think).
There are three records that we are able to access (one note just says it goes on for pages and pages.) Prisoner 36 succumbed after 9 days. Prisoner 20 succumbed after 18 days.
The longest we have a record of anyone else holding out is 100 days, and after that time they were taken over by a demon of passivity.
He and Spite aren't just working together. He's already learned how to move in order to take advantage of Spite's wings. That takes practice, collaboration, and patience.
He's mentally and physically strong enough to hold a demon back while Spite is furiously trying to stab someone at point blank range.
Basically? Outside of the Avvar, what Lucanis has done is basically unprecedented. He's also experienced and talented enough to adapt to an entirely new fighting style within the span of one year.