I too have searched for the .25 “unicorn” ratting pellet …
I use flatheads (RWS Supermag) in my .177 ratting rifles ….(Hobby’s) in my .22
So instinctively searched for the same for my .25
It’s well documented on here that all pellet types do the necessary if put in the right place….

However, for what it’s worth I find the flatheads do go some way into dumping more energy on impact and can go some way into reducing pass through …. Unless you are at exceptionally close range.
I remember the old days … as a juvenile putting pellets in backwards for ratting …
If you can get chance to shoot a domed pellet side by side same rifle with a flathead at the same spinner for example or at a solid surface, the difference in impact is notable.
So …. ideally a .25 flathead … yes please …. But it ain’t out there … anymore.
I tried the bisleys and the RWS Super H point … unfortunately not accurate enough for me.
Apparently JTS do a semi flat / dome in their .25 range … but I consider it too heavy for sub 12 …

I have tried the Hades and they are very good for accuracy … I am yet to try them on rats.
I am working my way through the Air Arms pellets and then have some FX to use … both super accurate and very efficient … the H&Nftt are lighter and will be “tested” once I have extinguished the above …
A good quality accurate .25 flathead at about 20gn ….. if only !