So far, the only way to get Uriel is to win him in an event or buy him for 2,300 gems. Please note, out of all the events, he's only really appeared 2-5 times.
Why don't you just make him a reward for level 70, or 75 or 80 etc. Surely it'd be nice to actually get a rare legendary monster after all our hard work instead of... one gem.
Also, go support this thread because it is EXTREMELY important (Man I'm telling you it'll literally save people hours of base design fails if it gets added in):