The Captain (Base) vs. Alexander Anderson (Human)
As a veteran fighter, Anderson is more experienced and polished than Seras, so he has the fighting capability that she lacks. Anderson can't hit as hard as The Captain, but it doesn't matter because it's unrelated to his fighting style. Anderson can match The Captain in speed and reactions, and although he's not as durable as The Captain, he makes up for it with his raw endurance and strong fighting spirit. Anderson is durable enough to tank a few of his punches since Teen Walter (who's not as tough as Anderson) was able to do it. It's unknown how he fares against his kicks, but it can be concluded that his arm won't split as Seras's arms did.
Anderson's advantage is that he has a plethora of anti-werewolf weaponry: blessed silver bayonets. He has ample supply and can spam them in quantities. As this is Captain's one and only weakness, his durability is not a problem for Anderson, who's tough enough to take Captain's attacks. He specializes in their use, which makes him a lethal opponent for captain. And given Anderson's speed in swinging and throwing his bayonets as well as their range, there'll be at least one attack that will tag The Captain.
The only problem is The Captain's mist-shifting, which makes his bayonets ineffective. However, he cannot attack while he's in mist-shifted, and Anderson will catch on to that quickly. So long as he has ample supply, tanking ability, and regeneration, Anderson's desire to win will give him the chance to catch Captain off guard and stab him in the heart, no matter how long or how worn out he'll be. Anderson's cunning and fighting spirit triumphs here.
Winner: Alexander Anderson with low-high difficulty.
The Captain (Base) vs. Alexander Anderson (Monster)
Anderson not only wields anti-werewolf swords, but his whole body becomes an anti-monster force. All of his physical stats increase, especially his attack speed and reflexes. In close combat, Captain would be quickly overwhelmed, as a full-power Alucard was. Outclassed in melee stats, Anderson would impale his heart before he can even mist-shift by blitzing him with a bayonet. His increased regeneration and endurance also safeguards Anderson's victory. The Captain would be dead before the vines coming out, and even if they do, they'll roast Captain into ashes.
Winner: Alexander Anderson with high-low difficulty.