From the Lowtown go to the central waterway area. Then do this:

(If you haven't done so already open the urn near the second Save
Crystal to get the candle that reveals hidden areas on your map.)

On your way through Lowtown, purchase three Black Belts. This is absolutely
imperative, regardless of level. You can buy them from the merchant sitting
in that lowered round area. They only cost 600 Gil apiece. (buy them but only equip them when the boss is about to use Disablega)

From the Lowtown entrance, here is how you find the Esper: head south to the
Save Crystal in the Central Waterway Control and use the Sluice Gate Key (Lost
in the Pudding Hunt reward) on the No. 3 Waterway Control so that both the No.
3 and No. 10 are closed, while the No. 4 and No. 11 are open.

Look at your map and proceed southwest to the No. 3 Cloaca Spur. Down near
the southern corner you will find a device called the No. 1 South Waterway
Control. Close that one. Now head back to the original controls.

You want the opposite effect of what you have now. Open numbers three and
ten, then close numbers four and eleven. Head for the No. 4 Cloaca Spur and
look for the No. 1 North Waterway Control at the end. Close that one too.
Now head back to the Save Crystal.

Make sure number four and number three are closed. Make sure the other two
are open. Now save your game and equip those Black Belts. Take the south
exit and run to the south end. Down one of the two staircases will be an open
door leading to a shallow square shaped area. Done.