This one's definitely a tough fight. The job-changes are ridiculously easy in comparison.
I had to upgrade my gear ( bought the lv 26 Grand Company set) and tossed a good food buff before the fight + Protes.

The thing is to keep the close ranged damage dealers to your Topaz and you have to focus on the Mages and the Archers first. Use Sustain to heal the pet if needed.

About Summoner, he gets the skills from THM and ARC. So you probably want to level those 2 classes as well.
Scholar gets CNJ and THM skills.