It was the same for Pursena's situation, too. If it were Eris, she wouldn't have tripped up just because the ground in front of her turned into mud. She'd properly keep her balance, or she'd stand still and get out of it.
Of course, there's no way Eris could do this in the beginning. Because she stacked up a lot of experience in battling me, she became able to cope with it. However, when I did something like that to Paul, he dealt with it even on the first time. As long as it's an advanced ranked swordsman with plenty of experience, they'd be able to avoid something like a bog.
Moreover, these days even monsters won't be caught by bogs. Even the stray dragon... Hm? The stray dragon got caught didn't it?