I haven't tried any in .25, but I'd guess the same ones that would work in the other Hatsan guns that are .25 would probably work in the Flash. I recommend getting the sample pack, and see if any of those work for you.
FWIW, squishy pellets are very good for hunting most critters. Generally speaking, you want the pellets to expand and dump as much energy as possible in the critter when hunting. You don't want to just poke a hole in it. The only time you really need that maximum penetration is on larger tougher critters like pigs, where you'd take a head shot that has to penetrate the skull.
Some of the hardest pellets that penetrate well would be the Barracuda Match, Benjamin Domes, and Eun Jin pellets. The JSB 34grain do pretty well too, since they are on the heavy side. The biggest benefit to the NSA slugs is how well the buck wind and maintain more energy down range. They are hollow points though, so they will expand in flesh. I hope that helps