About rewards:
https://hearthstone.fandom.com/wiki/Mercenaries/C%27Thun_(Event)
As for Ragnaros, you want him in a team like Chi-Ji, Geddon and Ragnaros. Maybe I’ll take a look if it’s doable with Malfurion, Bru’kan and Guff.
[Updated]
Okay, so, it can be a little close with the aforementioned combo, but doable on normal (and the party I’ve used is as recommended above: Malfurion, Guff, Bru’kan, Yu’lon with Pearl, Sinestra and Nefarian — you use either the nature trio or the dragons for normal opponents, depending on the colours and whatever is more comfortable, and the Nature party for the boss). It took me two attempts, and I’ve even tried to avoid most good treasures, so you don’t need to depend on them (but you can definitely grab them). Just make sure to use Muddy Footing on the blue guy, while Guff can work on Ravak, and Malfurion… well… is trying not to die, maybe even using Archdruid’s Call for healing.
Finally, there is an option I would generally not recommend, due to this self-styled ‘content creator’ and self-proclaimed ‘guru’ being, in my opinion, not so competent (albeit supercilious and toxic) as he thinks, to put it mildly, and his guides often being low-quality and misleading, but, if all else fails, you can try some of the options suggested here:
https://www.youtube.com/watch?v=gAyT7BoVpsI&list=PLAjYYYC68Q-kpW-RB9yBcvzwbGk2yKIDD&index=3
(there is another one below in the playlist)
Not something I would rely upon (it very well might depend on quirks and luck rather than repeatable strategy — I haven’t reviewed these in detail), but if even a player like this has managed to do the task with those parties, then so can probably you, and if it works — fine.
[UPD]
Out of boredom, just have done it also with Fire on Heroic without losing anybody. Couldn’t resist picking up Elementary Studies along the way, though (Ragnaros with Sulfuras instead of the Blazing Rune can be a completely different story sometimes), and my mercs are fully maxed. Balinda-Geddon-Ragnaros or whatever depending on enemy colours for climbing, Chi-Ji, Ragnaros with Sulfuras and Geddon for the boss, Alexstrasza and Tyrael on the bench (the former just in case, the latter for a chance of coins, which I didn’t get; you might be better of putting Cookie there, actually increasing your mercs’ survival chances).