It's possible to bait all the enemies outside of Joshua's range, and then have Natasha safely rescue her.
Even as a Pirate Ross' movement is rather low in water so it wouldn't have helped that much. Then again, Pirate Ross is infinitely more better than Fighter Ross since Berserker is far better than Warrior, and Garcia is a better Hero imo.
You'll be fine, this is one of the few FE games with buyable chest keys.
Anyways, I'd use Kyle, Tana (I'd personally go Wyvern but Falco also works), Neimi (Ranger), Gerik (I'd say Ranger, unless you drop Ross), Tethys, Cormag (go WK!), Ross (Hero,) and Saleh.
Most of the other units around this time are either mediocre (the other prepromotes like Innes or Dozla), they join too late to be really useful (Duessel on Eirika route for example) or they're absolutely horrendous (the other two trainees come to mind)
If you want to break the difficulty curve, just use the Tower of Valni and get everyone to level 20/20, now you'll have an invincible army. I'd advise against this because there's virtually no challenge anymore.