Use the Dragonator in its' lair. It has one on one of the upper levels. (3 Large spikes sticking out of the wall)
Also, Ceadeus will remain in its' lair after the main story ends, and the mission will not restart until you slay him, also his health remains the same as it was when the previous attempts end.
More or less, you have as much time as you want to finish the job. Sooner or later he will die.
If you get the chance, try and get a Rust or Ancient Shard (Online Tundra and Volcano quests): Rust are mined during normal quests and Ancient are mined during Elite Quests. They have a chance to give you Dragon Element Weapons which Ceadeus is weak to.
Also, Ceadeus will remain in its' lair after the main story ends, and the mission will not restart until you slay him, also his health remains the same as it was when the previous attempts end.
More or less, you have as much time as you want to finish the job. Sooner or later he will die.
If you get the chance, try and get a Rust or Ancient Shard (Online Tundra and Volcano quests): Rust are mined during normal quests and Ancient are mined during Elite Quests. They have a chance to give you Dragon Element Weapons which Ceadeus is weak to.