For this trophy, you will need to free the Green Sage, which is a requirement to complete the game regardless. After you've rescued the other Sages, you'll see a brief scene where the platforms to the Green Sage's generator align to create a path up.

Using high jumps (/ + ) or double-kick-jumps ( +  + ) will allow you to climb these platforms easily. After you reach the wooden ledge, there will be more spinning Precursor platforms coming out of the walls. Use timing to ascend these. 

Once you reach the top, the last obstacles is a series of spinning steps and platforms that rotate around the centre of the Citadel. Just wait patiently for platforms to reach you and you can jump up. There is a slanted platform that you may be concerned about, but you won't slide off; Jak is able to stand on it effectively. Once you reach the wooden walkway at the top of the Citadel, you can follow it around to collect all the orbs up there (See  The Super Orberator) and then break the generator next to the door to nab the Power Cell and trophy.

Note: If you have been following the guide and collecting all Power Cells up to this point for the  Maximum Power! trophy, then this will likely be the last Power Cell you need before the trophy pops, so expect to see that as well if you have 100 Power Cells at this point.