Abyssal F!Robin (PoR/RD limited) Low investment
Not necessarily F2P friendly because of the 5* locked heroes. But everyone just has their base kit + seals with the exception of Bike, who has Urvan refine and Reposition. This might be serviceable if you don't have all the heroes, as this approach uses a convenient choke point, so time isn't as critical as it seems. You can keep everyone except F!Robin up there by locking the healer (and later the red mage) right above Bike using a magic tank as bait. Just that should get you most of the way to a solution.
That being said Tibarn and Flychiah are reasonably popular, and I although I originally brought along Dance!Neph because this map is full of fliers, I didn't wind up actually using her for them, so assuming that Caldera Dance doesn't wind up being critically important, you might be able to replace her.
Tibarn is +Atk, but if you have a neutral he has his seal spot open for you to fix him.
My Bike is +1, I just don't have a standard Bike, sorry, you might need to give him summoner support instead to make up for it.
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