Sympathies, OP
It sounds horrible, and I experienced something similar once as a brand-new lecturer.
However, since your line manager (is it your head of dept or equivalent?) seems to be trying to duck responsibility for managing the problem, I think you need to get a paper trail started asap. A string of verbal complaints will mean nothing until there is an email record of your requests and their responses.
Would you consider emailing your HoD / line manager (whoever is at the top of the local tree) to lay out the situation as it currently stands (e.g., you are deeply troubled by Difficult Colleague's unacceptable behaviour towards you - give specific examples - and the situation has become untenable because it means you cannot perform your job effectively), and that you are not satisfied by the actions taken to date (e.g., previous discussions with Difficult Colleague had brought about no change in behaviour, or have made it worse; whatever).
Then you could request that your HoD takes immediate action, but give a choice:
Either
(A) Difficult Colleague is given a written instruction to behave professionally towards you, which will include ceasing to do X (specific examples) and cooperating with you to do Y (more specific examples). The instruction should include the proviso that if their unacceptable behaviour continues, you will lodge a formal complaint with HR.
Or
(B) Your teaching and admin load is arranged by HoD to minimise contact with Difficult Colleague (so one of you comes off the co-led module, you don't have to work on the same committees, etc. Be specific). This change should take effect immediately and for the foreseeable future, and should not result in you losing opportunities to develop your portfolio of skills for promotion.
You look forward to HoD's response to your proposals and would like a decision by [date about 8-10 working days away]...
A decent HoD would try option (A), or at least a close version of it, because it offers a potential long-term solution to a problem that is unlikely to be confined to you.
A completely chickenshit HoD who can't actually manage people will go for option (B), particularly if they are afraid of Difficult Colleague themselves.
But even (B) might make your life workable again... And you might have a better chance of a workable result if you try emailing a written request within the department (i.e., where HoD is obligated to respond) before going nuclear with a formal complaint to HR.