Going by ROTS: ICS The weaker Mandator II Star Dreadnaught reactor output is 7.6E26 W or approx 76 times that of an ISD. Not surprising as it can fight up to a thousand light Star Destroyers on equal terms. Dr Saxton estimates that the Executer class's reactor output is 1.3E27 W or around 130 times that of an ISD!
Which he estimates outputs 1.0E25W. An Executer would still have a power output 13 times that of a 10 ISD flotilla. ROTS:ICS also states that a true warship can divert nearly its total reactor output to her main guns. Take the 5,000 turbolasers and ion cannons from Inside the Worlds gives it over; and each would yield 2.3E23 W.
10 ISD's vs. the Executor:
Even after charging they’re capacitors for 10 seconds with their full power output the still don't match the Executors firepower. While during that time she will have gotten off 10 full power broadsides. Unless they Executor's captain is grossly incompetent, or under orders that limit his ability to maneuver of return fire; they just can't do it. After all three of them ramming the shields at high sublight can only just collapse the shields. According to ROTS:ICS a Munificent-class Star Frigate can charge it twin heavy turbolasers to a 2.8E26 J in about 22 minutes of charge time. So, 30-40 ISD's employing solid tactics might be enough, if a good number and hold back and overcharge their weapons. They'll still take heavy losses, as some will have to intentionally draw fire, and they'll also need to physically block her path to prevent an escape to hyperspace.
Loss of the Executor:
1. The Executor like all the other imperial capital ships, do not immediately engage the rebel fleet. They are ordered to hold back and take hits because of it.
2. Since the rebel are not limited by stupid command decisions that can concentrate their fire on the Executer without fear of reprisal.
3. The imperials allowed the rebels to close to point blank range before returning fire.
4. There were (3) Home One class Star Cruisers present each 3.8 km long and each at least 10 times as powerful as an ISD.
5. There were 6-10 Mon Cal cruisers, that if the ROTJ sketchbook is to be believed range from 1.2 km to 2.3 km in length; all of which were at least comparable to an ISD.
6. There were at least a dozen other lesser combatants.
7. There were an unspecified number of robot ramships made from converted Gallofree transports.
8. Only the bridge shields are known to have failed, temporarily.
9. The rebels could have held some vessels in reserve to overcharged their turbolasers. A single Home-1 type vessel with an output of approx 1.0E26, after charging up one minute could output a weapons yield of 6.0E27 J. More than 4 times the Executor’s total output and thus more than her peak dissipation rate.
Loss of the Lusankya:
1. The vessel's state of maintenance and provisioning are unknown, but unlikely to be anywhere near peak status.
2. The vessel was modified to serve as a hidden prison, thus may have been modified or may even had systems removed.
3. The vessel was possibly under crewed and captained by an imbecile.
4. The shields were dropped by 80 torpedoes of unspecified size and yield by freighters of unspecified class.
"The enemy outnumbers us a paltry three to one. Good odds for any Greek...."
"Spartans. Ready your breakfast and eat hearty--For tonight we dine in hell!" ~ King Leonidas of Sparta.