Despite how strong they usually are, the dragons are very weak at the start, so for a real challenge it is recommended to go 2000 blocks or higher if you want to use more bullets on them.
The dragons spawn in order from where they appear in survival and endurance as so:
|Fire Dragon||The first to appear in Dragon Endurance|
|Forest Dragon||Appears second|
|Sand Dragon||Appears third|
|Frost Dragon||Appears fourth|
|Undead Dragon||Last to appear then the cycle repeats itself.|
As of update 1.31, Dragon Endurance is now unlocked without the need of killing the undead dragon in endurance.