Welsh National Opera (WNO) sacked an oboist because his playing risked its world reputation, a tribunal has heard.
Murray "Sandy" Johnston, 61, claims he was bullied and harassed by former musical director Carlo Rizzi, 50.
The Italian conductor was branded a "vindictive prima donna" at the employment tribunal in Cardiff.
Jason French-Williams, defending the WNO, said it was vital that its performers were capable of playing at a very high level.
Mr Johnston, who had served with the WNO for 34 years, claimed wrongful dismissal alleging Mr Rizzi bullied and "persistently abused" him in a 16-year vendetta.
Giving evidence on Wednesday, he insisted any problems with the orchestra's wind section were not his fault alone.
But on Thursday Mr French-Williams told tribunal judges: "We have heard from the respondent in evidence that it considered itself to be a world-level company.
"It follows, therefore, that it is vital that the performers who play for that company are capable of playing at the very high level."
He said failure to remove somebody whose playing had fallen below the required level put the company's reputation at risk.
He also urged the tribunal to dismiss expert evidence from musicians who claimed Mr Johnston's ability remained very high.
Mr French-Williams said only people who heard him perform on a daily basis, such as those within the company, could judge.
Tribunal judges adjourned proceedings after completion of the hearing, reserving judgment.
But in his closing remarks Nick Smith, acting for Mr Johnston, hit out at Mr Rizzi.
He also accused WNO executives of failing to properly investigate the oboist's legitimate complaints of "harassment and bullying".
He described Mr Rizzi, who is no longer WNO musical director and who did not give evidence at the tribunal, as a "vindictive prima donna".
Mr Johnston had described how he got off to a bad start with Mr Rizzi during the conductor's first stint as WNO musical director in 1994, when he asked him to stop singing at a rehearsal.
A decade later, when he rejoined the WNO as musical director, he had told Mr Johnston to take paid leave to improve a perceived problem with his playing.
"The problem is that he stood up on occasion to Mr Rizzi and he has paid a terrible price for that," Mr Smith told the tribunal.
"The reality is that he has been punished on a number of occasions, including being withdrawn from the Katya Kabanova recording contract," for which he would have received an £850 fee.
"It is scandalous and spiteful."
He said on another occasion Mr Rizzi ordered the oboist to stop using a "mute", a soft object used to muffle sound, on his instrument.
"It is a bit like banning your carpenter from using his chisel," Mr Smith added.
He said Mr Rizzi's attitude to Mr Johnson had been "I am the boss and you do what I say".