How to beat Fallen (F) Robin on Infernal!?

Have you tried the F2P solution that uses L!Ike, B!Lyn/Nino and two Olivias ?

If you have Bridal Cordelia, I was able to kind of solo with her along with a couple of dancers. Some of the dancers did end up having to fight though.

I pretty much gave up. I've seen some videos on how to do it, and some of them use a Neutral Brave Lyn (which I don't have). I've tried practically every combination I can under the sun and it doesn't seem to work. I've wasted approximately 5 stamina potions on this, and I've had just about enough.

I'll just have to live without the extra 4 orbs.

if you have a ward armor team, you can play it defensively and barely loose any health unless you have a color disadvantage

i did end up having to use a psuedo-healer because of the reinforcements and constant color disadvantage

my H.Henry has Dancer's Ring because i had a spare inigo so he serves as my armor emblem healer sometimes when i find a map doing too much damage to my armor team but if you had azama or sakura they can work as psuedo-armor healers and can provide reasonable buffs if you gave them the drive def sacred seal and combined that with still-water balm but you do have to be careful with them because they wont benefit from the ward armor aura.

I heavily recommend a dancer, brave bow user (doesn't have to be a quad build, just high atk), and a tanky armored unit w/ wary fighter.
Just in case you have these units, I used:

+atk -res Sanaki +1 -- any high atk/res unit would work here
+def -spd LA!Hector -- Sheena/BK/Hector would work well too
+atk -spd Klein -- B!Lyn, Bridal Cordelia, especially w/ brave bow
4* olivia -- any and all dancers would work here
^^(Preferably, I would use a 4 star dancer even if you have a 5 star dancer, just to control enemy movement better)

Some general tips/examples:
Take out the troubadour as soon as possible
take note of every reinforcement, place units preemptively to take care of them
I put astra onto Klein to ORKO F Grima
I let my archer take care of the right side of the map in general, w/ my dancer helping him clear all of the units on that side
If using an armored unit, KO the Hammer+ armored unit to the top left as soon as possible

Hope this has helped in some way!

Yeah it is really hard i ended up using 10+ stamina potion until i actually succed, my team is jaffar, julia, mist, and ninian. For a tip make sure your team have someone that can either tank grima or kill her in one shot. I actually use another team and ended up using 4 stamina potion until i realize that none of then can actually kill her :p