Dawntrail's mediatour dungeon was already cleared without healers.

[MagiusNecros]

If you have a party of 4, can you do roulettes on any job choice?

I think even with pre-mades to qualify for roulette/bonus you need the standard composition.

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That said if Healers want to go on a massive strike they will need to:

Make a Global Online Petition

Communicate with Linkshells, Servers and Data Centers to coordinate a strike

Refuse to play the Healer Role

Unless a good percentage(millions) stops playing Healers I don't expect much to happen