So I played all the way through Shining Force II on the Genesis Mini 2 on Super difficulty, which due to a weird quirk, is the hardest difficulty in the game - for whatever reason, enemies don't do more damage than usual in Nightmare difficulty, whereas in Super mode they get better AI and higher damage. I think the only thing in Nightmare mode that's different is enemies have a much higher chance of dodging your attacks and also getting double attacks, both of which seemed to occur at a normal rate in Super mode.
It was tough! But very, very fun, and almost felt like a whole new game. Physical attackers are super dangerous for pretty much the whole game. Usually I just worry about getting hit by enemies with magic, but now, I was much more concerned about getting physically attacked, even by bosses. For pretty much the whole game, all your units can and will be one shot by non-magic attacks, so you have to be very careful about strategy, and you can't let anyone with low HP or defense get hit, ever, or they'll die. Even Jaha, usually my tankiest unit, could barely take a single normal attack from a regular enemy for most of the game, usually having single digit HP left after being attacked. All of this made the game much more about getting through battles, rather than managing experience levels or anything like that - you really can't, if you played it like I did, where you do no grinding and don't wait a ton of levels to promote (other than Chester, I promoted everyone either at level 20 or 21, 20 being the earliest level you can promote units in SF2. I only didn't do that with Chester because I wanted him to be a Pegasus Knight, which requires an item you have to wait quite a while for. IIRC, I promoted him at about level 28, which is far below what most people recommend for Super difficulty anyway, that being like level 40 pre-promotion for everyone. Not necessary!). I did use Karna, who has the Boost ability, which increases defense and agility for anyone in range, which also makes her easy to level up, too, but it was pretty natural because I basically had to boost my front line units to survive long enough to win battles. Without Karna, I would say leveling a lot pre-promotion would be necessary, but with her? It's a fun, challenging experience, where each move has to be carefully considered if you want to get through battles with minimum losses. I spent a ton of cash on reviving people, because one errant enemy could kill someone in one attack.
Because of all this, I used characters I usually don't in this run - I retired Gerhalt and Luke as soon as I could, since their attacks are so weak and they're both pretty fragile, and used Skreech for maybe a quarter of the game, until he got left behind in levels, when I replaced him with Sheela, who I almost never use, since she was another decent attacker with healing spells (though she learned Heal 3 in literally the last battle, so she never saved anyone's life, just topped them up lol). I even used Chaz, mainly because Higins, who himself had replaced Rick, was just not keeping up damagewise. He's actually pretty decent!
All of this is to say that the game is certainly possible, even on the hardest difficulties, without grinding. I told myself I'd only use Egress if I got into one of those pesky repeat battles, or if I forgot to revive people before a battle, and I stuck to that and got through the game just fine. Really had a great time.
Now to finally play all the way through Shining Force CD.