Senior co-author is jeopardizing two years of work I have been working on a certain paper for about two years, the first year funded and the other one pro bono.

The data and methods were fixed during the first year. The data has high quality, the sample size is three times larger than in a typical study, and the papers that describe the methods I am using have been cited thousands of times.

Now one of my senior co-authors, for some reason that I do not know, is saying that the methods are essentially junk, and refuses to allow me to submit the paper to a good journal, though he would find low-rank journals just fine! I have rewritten the paper once already but it still does not satisfy him. The other seven authors have given generally positive feedback with good suggestions on how to improve it.

I will soon finish my studies, so I have little incentive to remake everything for a third time. However, it would be wasteful to discard the paper entirely. I don't know. I am very angry about this but I do not know what to do. My supervisors have been helpful but I feel I am not getting anywhere.

I could perhaps file a formal complaint and burn some bridges but then I would only lose more time in that. Any suggestions for further action?
I suggest that you get all of the co-authors together to have a discussion. When you organise the meeting, say something neutral like "let's get together to figure out what to with this paper".

If your senior co-author has a good point about the methodology being flawed, then the other co-authors may suggest a way to salvage the paper. They also have an incentive to get the paper published, if possible.

If the other co-authors don't agree with the senior co-author, then they can put forward their arguments, and hopefully a resolution will be reached.