The first one is: 气息一沉 I am kinda confused. Does it mean that the person has exhaled heavily (like when you do a Karate kiai) or does it mean that the person's aura has become denser or something (like when Kenpachi showed off his Reiatsu to Ichigo)? The second one is: 滚刀肉 I checked Baike and other online dictionaries. They all say it refers to a difficult person. Someone who's difficult to shake off. But could it possibly have another meaning? Like for example, could it refer to someone who's used to killing?
Blind guess because need more context: 气息一沉 likely means that moment when the character pauses/calms down/turns serious right before doing something momentous, like opening the door to a room with something scary in it (cheating significant other, actual horror things, etc) 滚刀肉 refers to meat that is either so inferior in quality or had way too much skin and tendons attached to it that it is hard to cut, cook or eat. When translated to slang, it would mean some insistent sleaze who won't leave you alone. Basically, that person is the meat mentioned. Shitty human who's hard to get away from, get rid of or rationalize with. Imagine the stereotype of the "shitty middle-aged sleazy bum who keeps harassing women even if everyone around him tells him not to ". Would seriously need more context as to why you think it might refer to someone who kills a lot. (some writers love using their slangs in weird ways)
Ah sorry for not providing context. I went to bed right after posting. The first one. 他气息一沉，气运丹田。而后，忽然发力，一拳打向巨树！ The second one. 像他那种只会打擂台的拳手，一遇到龙哥他这种滚刀肉的，一招必败！ Previously, they were saying that this 龙哥 was a mercenary king who had killed thousands on the battlefield. Basically, he 杀人过千. That's why I thought 滚刀肉 means "experienced killer". But I guess, based on your explanation, the meaning is closer to a "difficult opponent"?
For the first, you can say he "focused, gathering his Chi/Qi". You can also say he "calmed down and focused". Depends on what he was feeling/doing before. On the second, it's more like a cheater/street brawler who knows no rules aka sleazy thug lord. Translation: that guy has only ever competed in formal duels before, how could he win in a proper fight against *insert name*, a cunning mercenary king? He'll surely lose.
他气息一沉，气运丹田。而后，忽然发力，一拳打向巨树！ I have yet learn any qigong since it's ban in China. But I try. 丹田 is a part of your body just below your chest. On you belly. There is 4 points(穴位) there are some books on certain sickness that use those points. But I as average Chinese only knows few points in Chinese 督脉 have no idea where it is other than its have some thing to do with blood circulation. So basically I will translate as. He lowered his qi to around his lower-upper-body around the abs, and cuirculate the blood with his qi, he than instantly use force, knocking down a big tree. Really, it sounds awful. @Shibb idea is better I think.
像他那种只会打擂台的拳手，一遇到龙哥他这种滚刀肉的，一招必败！ why do i read this line and I think it's from some other areas. But @Shibb is right. Or wrong at the same time. I mean it's correct, but I think 混刀肉 it's mostly means 很难伺候的人 that just my understanding of word. Not the author understanding of the word.