So I ended up taking my own advice and trying out my NG+ LLG idea. But instead of making all my units lv 1, I made them join at their starting levels + stats, which would be:
Crono lv 1
Marle lv 1
Lucca lv 2
Frog lv 5
Robo lv 10
Ayla lv 18
Magus lv 37
That's a party average of 6.17 before Magus joins, and 10.57 after he joins. In contrast, in a regular LLG the party average is 11.83 before Magus joins and 15.43 after he joins. On the upside, I have access to all equipment, techs and items, and allowed myself to max out everyone's speed using speed tabs. However, I banned the use of power and magic tabs for obvious reasons.
Well, as it turns out this challenge was a breeze for the most part. The rainbow sword alone can carry the team's offense for almost half the game. A lot of double/triple techs are broken even under an LLG restriction, in fact I'll name them:
Beast toss
Falcon hit
Twister
Drop kick
3D attack
Spin strike (requires silver rock)
Twister is especially ridiculous. I was clearing out entire groups of enemies in the tyrano lair and ocean palace using this attack. Even on sidequests it's still clearing most enemies in 1 hit. I thought random battles would be a chore in this kind of challenge, but they're actually as trivial as on a lv 50 team because of this double tech.
Another thing that kills this challenge, max speed + haste helm simply makes your team too fast to ever get into serious trouble. At times I was fitting in 3 triple techs before the boss could even react. Think how much time that gives me to revive dead units and use a lapis for healing. Also, many battles can be impossible to lose with the right choice of mail. For instance, red mail turns Tyrano, Rust Tyrano and Golem Twins into a joke, just to name a few.
I can only remember 3 battles that were truly challenging. Heckran was tricky because Marle and Lucca's magic damage was absolute rubbish. I mean antipod 3 was doing 300 damage. Still beats the 50-70 damage that Crono does from luminaire, heh :p Magus was a really fun fight, took a lot of thought to come up with a viable strategy. Super Slash, Flea Plus and Great Ozzie were also pretty challenging.
I abandoned this challenge after a few sidequests because it's just too easy without making further restrictions (eg/ no Robo or Ayla). Defensively, no boss could possibly defeat my team because I could always make a team of Crono, Robo and Marle that use lifeline, and use megaelixirs to heal. It would just be a matter of time before uzzi punches and rainbow sword crits take out the boss.