With the Watcher, this can be done very easily with one card - Study
Below is my deck when I got this achievement. I found study early on in a game so was able to do this against a weak enemy.
Study gives you a zero cost card every turn called Insight. Insight is retained in your hand and draws two cards for free. All you need is a way to survive long enough to get 25 of these cards in your hand. For me, that was to use the power card Like Water which gave me 5 block every turn I was in calm stance, and versus one of the little bugs in the first area, 5 was the most damage it could do. You will either need a way to block each turn like this, or fight something that does little enough damage that you can tank the hits until you have enough cards.
I skipped my turn until I had a full hand of insights and 15-20 in my draw pile then spammed Insight cards until the achievement popped. If you drawvany other cards in your hand, try to use them to make space for insight, it is possible to draw a full hand of other cards and not have any insights left make sure you draw enough to get the achievement, for this reason, I recommend taking turns to get more than 25 of these cards.