Supposedly shieldstun works with an equation of the form
stun = damage*ratio + constant
However, it is suspected (and very likely) that there are two pairs of ratio/constant - one for attacks which are designated 'weak', and one for those which are 'strong'.
Somewhere in this thread there is frame data provided by Magus for a variety of attacks performed by Luigi, ranging in damage from 0->25%.
EDIT: If it's not too much to ask, I'd like to have PW's contact details please, Kupo. I'm working on some jumping codes and I'll have to make use of some values which the system will keep track of for me (They're called Gecko Registers to programmers). However, I know PW makes use of them too, and I'd like to know which ones are available for me to use.
EDIT again: Found the data. Wow, I'm not used to forums where the search function is actually good.
Character . Attack .... Damage .... Hitlag .... Shield Stun
-----------------------------------------------------------
Luigi ..... Up-B Wk ... 1 ......... 5 ......... 0
Luigi ..... Jab 2 ..... 2 ......... 5 ......... 1
Luigi ..... Jab 1 ..... 3 ......... 6 ......... 2
Luigi ..... Down-B 5 .. 4 ......... 6 ......... 2
Luigi ..... Jab 3 ..... 5 ......... 6 ......... 2
Luigi ..... N-B ....... 6 ......... 7 ......... 2
Luigi ..... U-Tilt .... 9 ......... 8 ......... 3
Luigi ..... F-Tilt .... 10 ........ 8 ......... 3
Luigi ..... D-Air ..... 11 ........ 9 ......... 3
Luigi ..... B-Air ..... 12 ........ 9 ......... 4
Luigi ..... U-Air ..... 13 ........ 10 ........ 4
Luigi ..... N-Air ..... 14 ........ 10 ........ 4
Luigi ..... U-Fwd-Sm .. 15 ........ 10 ........ 5
Luigi ..... D-Smash ... 16 ........ 11 ........ 5
Luigi ..... Up-B Sw ... 25 ........ 14 ........ 8
Luigi ..... Side-B Ch . 26 ........ 15 ........ 8
Some means of reducing hitlag would be awesome too, by the way. I have no idea how to find those things, but if you give me the means I'll work on it.