Brewdog's 35% one (Tactical Nuclear Penguin) is done through a process of alcoholic concentration, they remove water in essence. And we don't talk about those techniques here... :thumb:
The others are all the same, Sink the Bismark, End of History, Armageddon etc etc.
Brewdog do however have the strongest fermented only beer. It's called Ghost Deer and it weighs in at 28%.
I believe that they effectively "trained" their own strains of yeast from other massive experimental brews up to the low 20%s. Making the actual beer requires multiple ferments on different yeasts each one hardened to working at a certain ABV...
To do this yourself you would need to plan a series of beers, each being racked and moved on at exactly the right times to keep the yeast working hard at every stage of the final 20 something percent beer. I worked out a plan to be able to do it but unless I win the lottery and can dedicate about 5 months to it... It's not going to happen...