caster's by their very nature fall off late game because of how spells and MR scale. and the only person ur ever gonna 1 combo with ap ez is someone with 0 MR. also using phase shift offensively is dangerous at best, suicidal at worst. he's not bad, but id put him tier 2 at best, mostly cuz of the long CD on his ap scaling spells.
In a recent game 18 lvl Varus with a typical AD Carry build + a Mercurial Scimitar that gives +45 mr, died to my E+W+Q from full health.
AP Ezreal has the fastest cooldown in the league to which only Ryze can really stand up:
Q - has a cd of 2.4 sec, 1.4 if you land that Q.
W - a cooldown of 5.4 sec, -1 sec for every Q you land, if you are good about 3.4 sec.
E - a cooldown of 6.6 sec, -1 sec for every Q you land, if you are good about 4.6 sec.
R - a cooldown of 48 sec, -1 sec for every Q you land, if you are in a constant fight and are good, about 25 sec.
Ezreal is a spell machine gun.
If you don't have 40% CDR, you are not being serious, because Ezreal benefits from it in Square with his Q.