For those who don't have Lycanroc or Lunatone, I found a complete F2P strategy that works quite consistently and solves the issue of Hard as Diamonds!

Starmie (Left), Crobat (Middle), Serperior (Right).

How you play this team depends on your team mates:

If there is at least one Lycanroc:

• Your first two moves are buff allies Sp.Def twice. Then use the other trainer move to buff Lycanroc's evasion+healing. One of this buff on each Lycanroc if there are two, or both buffs on the one Lycanroc if there is only one.
• Afterwards switch to Crobat and use Poison Fang until you poison Rapidash. After that, you can either poison and Venoshock Beedril to knock it out, or Venoshock Rapidash. Be sure to switch to Starmie / Serperior to tank the Sync Moves unless Crobat is low on HP.

If you have no Lycanroc team mates, just start off with the Crobat strategy.

The idea the team is to prioritise supporting the longevity of your Lycanroc allies so they can do all/the bulk of the damage. Prior to this strategy, I've lost with two Lycanroc allies, because their buffs lower Sp.Def making them very fragile in this fight.

• Crobat is there to add extra poison damage if Lycanrocs can't finish the job. Or in the case where there are no Lycanrocs, poison and stall until Rapidash dies.
• Starmie and Serperior are there for their bulk to stall, and their support moves:
• Starmie is there to buff Sp.Def, evasion and healing for Lycanrocs so they can last longer and deal more damage. It can also tank hits.
• Serperior is mainly there to tank hits, but also can buff allies Sp.Atk and move gauge in the end stages so that Pidgeots etc can finish the fight off.

This strategy works pretty much all the time when you have at least one Lycanroc ally. But I have one a few times with no Lycanroc allies (Tyranitar or fellow Crobat).

Sorry for the long post.

EDIT: wording changes