I think the best thing you can do is to ignore and carry on with your work as usual. To protect the juniors, you need to bring them under your ‘sphere of influence’ to shield them from the toxic junior eg by giving your trainees work so that your trainee can say they are busy with your work. Unfortunately, I think that as a fellow junior, there’s not much else you can do. You can try to tell your partners about it, and paint it as the toxic junior terrorizing the trainees but you would need some strong evidence or anecdotes to back you up. Good luck!