I'm going to answer, but please know that this isn't the only answer. The moves have triggers that take place in the narrative and how zoomed in and cinematic it is. The fiction decides. That said:
If we remove most fictional cinematics... and assume Dremmer and the PC are in the scene.
A) [PC]Bob vs Dremmer
Bob wants to kill Dremmer, Dremmer is prepared to fight back. The Move: Single Combat
B) [PC]Bob vs Dremmer
Bob wants to kill Dremmer, Dremmer is prepared to run the fuck away. The Move: Seize by Force or Cat and Mouse
How this works varies a lot depending on the complications Dremmer can involve in his escape. Please note that there should be complications unless Dremmer literally has no where to go. First you ask, who has the advantage. If the PC has the upper hand, I'd probably go straight to seize by force, where Dremmer escapes unless the PC either deals 3+ harm after armor or picks to seize definitely Dremmer's escape. 2harm, Dremmer escapes but probably wont be around much longer. 1harm Dremmer is in the wind. Cat and Mouse can follow this if appropriate, but if Bob catches him again, there might not need to be another seize by force to end him.
If Dremmer has the advantage, then Dremmer just Runs. In order to catch him Cat and Mouse happens (with Bob as the Cat). If Bob gets to pick where he catches him, then Dremmer's got no way out and it's Single Combat time. Otherwise if Dremmer gets to pick where, it might be seize by force or it might be something else, depending on where (or who) Dremmer gets to.
C) [PC]Bob vs Dremmer & Gang
Bob wants to kill Dremmer, Dremmer and gang are prepared to fight back. The Move: Single Combat
Single combat doesn't mean one person vs one person, it applies just as easily to one person vs one gang of people.
The PC does not get the option, I just kill Dremmer. They have to fight through bodies to kill him, and those bodies are actively getting in the way / fucking Bob up. Deal harm to the gang, and the leadership will start to crumble, the fiction comes in and asks are these boys ready to die for Dremmer? If so, then fight on, if not, maybe they scatter and now its PC vs Dremmer, refer to scenarios A or B.
D) [PC]Bob vs Dremmer & Gang
Bob wants to kill Dremmer, Dremmer uses his gang to help him escape. The Move: Seize by Force
Bob has to fight his way through to even START chasing after Dremmer. That's seize by force, definite hold is definitely fighting through the gang. Interestingly this is actually the best way for Dremmer to survive and tends to deal less damage to his gang, because the PC's focus is getting past them not kill all of them to take Dremmer down too. Still: Dremmer Runs. Bob has to fight through the gang, the gang will break if enough harm is dealt... but they might break in the same direction the PC wants to go, thus, to keep them out of the way, seizing a way through is essential. Followup, Cat and Mouse (see scenario B), unless the PC didnt make it through with one move, in that case, Dremmer's gone.
E) [PC]Bob vs Dremmer & Gang
Bob just wants to kill Dremmer, Dremmer fights or runs. The Move: Seize by Force under Fire
Seize Dremmer's escape (running), or seize a way through (fighting) the gang to Dremmer, while acting under fire of all the people trying to stop Bob. Re: old post. I would probably play scenario C and D more often then not. This is AW, people in the way of killing, probably get killed. If the gang is ready and staring you down, you're probably not going to assassinate the guy out of the middle of them without serious Cool rolls. And even if you do, you have to Cool/Hard you way back out too. Note: I, after thinking it over, REALLY dislike this option.
PS
The thing to note here is that if there are no other complications in the field, no people getting in the way, positioning or tactical shit that matters, and the killing needs a roll, it's probably single combat. If they cannot run, they will fight, and a cornered person fights much harder then one who can run. Same goes for a gang. If they're trapped, they will fight until they're all dead. If Bob is attacking a gang of people, Dremmer can use that gang's health pool to stay alive all the way until it either cracks under pressure or is crushed. 5(maybe4) harm to the gang is when I'd deal 1-harm to the Dremmer. 6(maybe5) harm to the gang, Dremmer's dying in there too. 6+ Dremmer's been dead.
This actually makes the battle babe even cooler. Visions of Death: Dremmer Dies. Battlebabe hits the gang with one exchange of harm (preferably from go aggro, but seize by force too) and chooses all the get away, suffer less harm, frightens them options. Because, the battle babe already killed Dremmer out from the middle of them (visions of death) and only had to trade harm to make sure the battle happened, so her focus is actually on escaping.
That's my 2-cents.