He's mid to high kage level, He fought off gai's team with kakashi's team with no arms and still managed to use a clay clone to get away and were it not for kamui he'd have killed both teams in 1 shot. He can see thru genjutsu and even though he specializes in long range he can still prove formidable in close to mid range. His bombs can do everything from swim to fly meaning he can fight at his full potential on land sea or air, and his c4 could even 1 shot someone on Hashirama's level. Not too many mid kage level ppl have that kind of destructive power.
Furthermore he's smart enough to deduce the mechanics behind Gaara's sand and even gave up an arm to land successful counter against someone who's defense was lauded as perfect not to mention he'd have been able to kill him in less time. He's definitely not high kage level compared to someone like Hashirama, but this doesn't change the fact that he's one of the few ppl who could take him out with no difficulty if he succeeds with c4.
Deidara has it all to the point that he can combat a wide variety of opponents and he did kill sasuke, it's just that plot enabled him to get away using Manda even after he was supposedly out of chakra. But none the less he can combat a wider variety of ppl than most mid kage leveled shinobi. this to me means he's bordering on the edge of high kage level, the only thing he lacks is a hax defense.