Lona is pretty weak. She's struggling with using a broadsword (it's probably a one-handed one, she probably wouldn't even lift a two-handed one), what do you expect her to do against 4 grown men? She's easily knocked-out in the game several times, so 4 soldiers probably can easily dispatch her. She can clear monsters and bandits because she has healing items (potions), she's usually fighting one or two at a time, and she's more of an agile fighter than a strength one. Lona is a malnourished short girl, after all.
Edit: i've just realised that berserker's sword is actually a two-handed greatsword, but it doesn't prove anything. Realistically, she wouldn't be able to use it at all,(cuz it's a reference to guts sword, no way she's lifting it), but even if she can, it doesn't change her durability or endurance. She has VERY little stamina, she's easily knocked out.