Pronunciation of "Ozymandias"

Solution 1:

English verse is qualitative, not quantitative: it runs stress-to-stress rather than syllable-to-syllable, so an extra reduced syllable here or there is negligible.

Moreover, English poets have great freedom to play against strict "meter" for local rhythmic effect. The pentameter line in the English tradition is predominantly iambic, but variation is permitted at any point: any foot may be realized with what strict metric would regard as a trochee or anapaest or dactyl or even a spondee.

And in fact the pentameter line is only formally pentameter: there is a continuous wrestling match between the nominal five "feet" and the underlying four-beat rhythm. Consider for instance Prospero's line from The Tempest:

In the dark backward and abysm of time.

That's impossible to scan as even approximately iambic pentameter without grave deviation from ordinary speech rhythms. But as a four-beat line it's perfectly natural, and does not feel at all out of place.

_{in the} dark back_{ward and a}bys_{m of}time.

Shelley's line is not nearly so deviant as this. Shelly does not depart radically from measure or openly assert the dominance of beat, he merely takes advantage of the contrast between the opening proclamative long syllables (almost a spondee) and the long sequence of short syllables in the polysyllabic name to 'fuzz' the collapse of the 1-2-3 stresses into two beats.

Solution 2:

I like

/ɒ zi ˈmæn di əs/.

You can find audio tracks of this in Oxford. The second pronunciation stresses the first syllable, so OZ-ymandias.