Just throwing it out there, but the Ascendant Spear's surprise advance strike on Coruscant wouldn't work, what with the planetary shielding and Golan battlestations and whatnot.
Oh lord, what crazy weapon have the EU writers conjured up this time....*searches Wookieepedia*....oh not to bad, just like a heavily armed space station. However I also discovered that Coruscant has a four-tiered defence system, so this seems a good a chance as any to see how the SE could win this fight, as too kill Sidious they would have to get past them.
1st Tier: Space mines - these can be deactivated from below, an infiltrator or a brainwashed member of the GE could easily achieve this (e.g. Mara Jade) However I'm not sure these existed until the New Republic era as they are only present in the Battle of Coruscant and the Lusankya managed to escape Coruscant without encountering them - despite it being under the control of the Rebel Alliance.
2nd Tier: Golan Defense Platforms - which you spoke of. But I'm afraid they wouldn't last long against the Spear as they seem to be only equal/slightly stronger than an SD. Still it delays the bombardment on the Imperial Palace a little while longer. Allowing more time for Sidious to get to safety and/or for the ambush to arrive.
3rd Tier: Planetary shield - can be deactivated much like during the Sacking or Coruscant again by an infiltrator or one of the GE's own.
4th Tier: Turbolasers - inconsequential seeing as the SE won't enter the planetary atmosphere.