He was already super weakened after all the tragedies. He was like a Yonkou Commander I would say. He fought Kizaru for a while but got desperate soon and resorted to that explosive substance.
He was very weakened from everything that happened to him (will power = Haki) so his main weapon was a lot weaker than in his prime. Also, he was older than Garp and WB if I'm not mistaken. Oda said in an SBS that the only reason Luffy could fight him was that he was an ex-admiral, not a current admiral.
The funny thing is i haven't watched the movie... 
But what i mean is... you can make any kind of explanation like "he isn't at his prime", but i still... don't see luffy defeating an ex-admiral, it's not like that much time has passed, i don't know, i just don't like to mess with those things. The manga itself has a weird power-level classification, messing with it adding characters which sometimes are cannon, someties not, sometimes the chair they broke is cannon but they aren't... it just messes thing up to me 
I hear you, that isn't a flaw that is a common reaction to those lame movies. they suck.
luffy beat the golden lion? ya right...... that man was insane, he fought garp and sengoku in his prime. "he was old and sick" ya ok, unlike the manga characters movie characters have the unique ability to become weaker with age...... WB was still the strongest..... admirals are old as hell..... but zypher? nope that is too old, he is a weakling now.
that and also their powers are stupid....there was even a water logia that they put in a bottle...... my general stance on logia is if they can not get out of a bottle/barrel they are worthless and should of never been made in the first place....
That's exactly my point!! I mean... WB wasn't at his prime, was incredibly sick, and a lot of things happened, one of his "sons" was going to be executed, even an ally attacked him, and you know... willpower=haki and alll that crap. And a lot of years have passed since he was in his prime, but he was still a monster!! I mean... 
And you forgot another one: They waste interesting DF powers sometimes (which is a reason to hate the anime for me), or make a new DF appearing in the manga look like a copy of an anime one... i hate that... (maybe that's why they use stupid powers sometimes, to make sure that doesnt happen? 
)
Not to mention Zephyr's battle with Luffy isn't canon, just his history in the Marines.
I don't like that either... I mean, make it canon, or don't do it, but mixing non-canon and canon just makes it even more confussing... 
