I hate to say this, due to how much I fanboy over Guts, but Seras got this, with some difficulty.
On a close-range fight, she wouldn't stand much of a chance. Of course, she may be stronger, but Guts' skill with his sword, immunity to pain, lack of self-care and Dragonslayer's haxxy nature would surely make him win.
But they're fighting on quite a distance, and if she has full knowledge about Guts, she'll be sure as hell to stay away from him, sniping him while backtracking. If she stayed in one spot in the air, I have no doubts that Guts would eventually reach her (After all, he can leap to absolutely ridiculous heights with the Berserker Armor, and his agility is also incredible). But even then, assuming that she'd miss every shot and not at least heavily maim Guts to the point she can keep up with him physically is kinda unfair. So, either way, she backtracks and snipes him to death. The cannonballs, crossbow bolts and thrown knives would put quite a dent on her, but they'd be temporary at best and not really all that good to finish her off.
