3 hours ago, Ghostofman said:A 20mm autocannon, isn't a 105mm howitzer, isn't an 80 cm railway cannon.
The cannon in ESB was something different with it's own ridiculous statblock that I don't think we've seen yet in official supplements.
It takes a max of 12 ion torpedoes (found in FO) to take a ISD from 0 Strain, to disabled.
In the case of Rogue One, that scene was probably a Mass Combat check (since the players were all somewhere else doing something else) so saying a single squadron of Y-wings making an ion torpedo attack on ISD results in the ISD being disabled is easily explained.
If the players were directly partaking, 6 one-seater Y-wings all piloted by players successfully firing Linked 1 ion torpedoes could do it in one pass.
Of course, in the RAW of the game, Y-wings can't even carry ion torpedoes, so it's even more impossible to replicate what we see on-screen... And I'm OK with that.