We’d have to surprise it, and we’d have to be spread out, so the dragon breath could only catch one person, with two people able to be caught in it who are prepared in some way with either resistances or immunities, presenting a tempting target (since both can be hit at once). Then, we’d need a way to restrain it, either with grappling or something, preferably while it is prone so it can’t get up. Having a Paladin to help counter the Fear would be nice, and if course, everyone would need to have full resources.
Restrained, prone, having used up its breath and making the Fear useless, the surprise round would be used making it as impotent as possible, and then its a matter of using all the nova abilities, preferably after its cursed so it has disadvantage on whatever saving throw would be most useful. Even with Legendary Action Attacks, if it can only attack those close to it, then we make sure those targets are limited. Drop an anti magic field around it so it can’t cast spells, and we’re golden.
And Primus can grapple with his mind.