I have no idea what you mean, you just saying you think warcry is bad?
Hes just spamming to get posts.
I never use act5 merc so i dunno. But he probably has enough ar to go along, so the only way to maxmize strike chance is -def% (ignore def works only on white mobs so thats not an issue anyway).
Now you need to calculate does he strike more often with - 25% def or with 1 more attack per second and thats that.
But in my experience more attacks usually do the trick better.