There are actually 2 seperate areas to enhance defender selection. One of them is specifically for countering Marksman, and another one just changes what it counts your base strength as when selecting defenders. So you could make it use the one and not the other to make them fodder for regular unit attacks.
Catch is though, if you have it select Mr. Weakling, then you are feeding your opponent XP. So you may find that you do not much like the change if it were made.