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There are three cards that give you Focus: Defragment, Consume, and Biased Condition.
I recommend aiming for this achievement with a defensive build, because Lightning + high Focus usually kills the enemies before you can reach 25 Focus.
Personally, I was able to unlock this achievement by using Creative AI. Creative AI generates a random Power at the start of each turn. Combined with Echo Form, it can even generate 2 Power cards per turn. This means that you only need to have enough Frost to survive until you get 25 Focus.
Upgraded Biased Condition gives you 5 Focus at the cost of losing 1 Focus each turn. But it can also be combined with Core Surge (Panacea or Ancient Potion work as well) to nullify the negative effect of Biased Condition.