I haven't seen any evidence anyone can beat Construct 2's automatic spritesheeting and PNG recompression with other tools. If you can show me something that beats it by a significant margin I'd be interested to know which tools you used. As the blog @Knifegrinder linked to says, C2's automatic spritesheeting on export is very good, and there is not much to gain by doing anything differently. For example putting two completely unrelated images in the same spritesheet usually has little compression benefit, because there is not anything in common between the images that PNG can compress away. Compare that to animation frames, where there is often a lot of in-common image sections which can be compressed away, and it's more likely that PNG-8 can be used for further savings, whereas unrelated images may nudge it over 256 colors and use PNG-32 which can double the filesize.
Also have a look at Image compression in Construct 2
, a separate post detailing Construct 2's PNG recompression strategies. It's fairly sophisticated and includes automatic color counting to losslessly switch to PNG-8 where possible, giving significant savings for free. People occasionally try to use online services or standalone tools like PNGCrush, but I have tested all the ones I can find and none of them beat the way Construct 2 uses several tools in tandem.