"By .75 times", that's multiplication. It's isn't "the amount by which damage goes down is 75%". "by" in his sentence is multiplication. I think it's generally common knowledge that the word "by" implies multiplication, anyone who's done a world problem in middle school should know that. The .75 is equal to 3/4. So you're multiplying the damage of a move by .75 which is going to equal 75% of what it would be to one pokemon. If he wanted to say it the way you're implying i'm sure he would've just said "to 75%" instead of .75 because there's no reason to bring decimals into this unless you're doing multiplication which was the case and you misunderstood him.Weakened by .75 (what was said) = the amount by which damage goes down is 75% (i.e. final damage is 25%). = Incorrect
Weakened to .75 (what he meant) = the amount of damage goes down to 75% of normal = Correct