I have a two-fold approach to this:

* With specific positive or negative feedback, I will issue friendly complements and critiques the LLM to reinforce things I like and reduce things I don't.

* Rather than thinking sycophantic/antagonistic, I am more clear about its role. e.g "You are the Not Invented Here technologist the CEO and CTO of FirmX will bring to our meeting tomorrow. Review my presentation and create a list of shortfalls or synergies and as well as possible questions".

So don't say "please suck at your job", give them a different job.