Power of nature can be implied through the fact nothing man creates is permanent; 'Half sunk a shattered visage lies, whose frown, and wrinkled lip, and sneer of cold command.'
- All that is left is the insignificant memories of Ozymandias' and the broken statue. This statue was left abandoned for a long time and the 'boundless' sand has caused it to wear away and be forgotten.
Power of man can be seen through 'king of kings', Ozymandias labelled himself as this, unaware of the fact his reign would be forgotten.Therefore it stresses the idea of mortality and the view that man inferior to the rest of the world,as an individuals mark is only temporary.
His statue as a whole is symbolic of the fact things disappear, this is further highlighted by the line 'Round the decay of that colossal wreck' the decay is the loss of his power in relation to the extreme power of nature.
Man is therefore not as powerful as nature as nature lasts forever and ruins what man makes, it is the downfall of man as a result. Obviously there are ore specific examples you could speak about but these are the broader topics the poem touches on.
In response to your original question power is presented in two main ways, I hope this helps