To avoid the boiling oil you need to capture the Gatehouse by moving troops on the wall onto it or destroying it...otherwise you have to enter the city by another way.
If you want to fight on the walls then Armored or Spartan Hoplites can beat any Pre-Marian Roman infantry but after the Marian Reforms you probably should use mainly Spartans for wall fighting. Training your troops at a settlement that has the Temple of Nike helps also helps as it boosts your troops' experience by up to 3 when you have the 4th Level of the Temple built.
The other way (probably the easier one) is to bring 3 or so units of Onagers and destroy the Gatehouse (you need to destroy both the door and the walls above it to turn off the boiling oil) and some wall sections as well as all nearby towers. If you sap the walls you can reduce the number of Onager units to 1 or 2 but relying on only sapping will result in alot of casualties from the towers if the defenders can put up a tough fight within their range. Sapping also isn't available for Epic Stone Walls.
Btw, if you send a spy into the city before attacking it there's a chance he'll open the gates when you attack it, if he does that then the boiling oil is turned off.
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