I don't really get what the OP is asking for here. Make all the fights easier so people don't quit?

There's basically only 2 ways to make boss fights difficult, gear checks and mechanics. Remove the mechanics part and all you have is gear checks which can eventually be overcome by anyone.

As an example Turns 6 and 7 are very much mechanics based fights, you could probably beat them both in i80 gear, possibly lower. If you do Turn 6 in a certain way (Feed 1 slug to each slime) then there are basically zero dps checks on that fight and you can take as long as you want (maybe the bee but it still takes a while before it starts one shotting people). The only real dps checks in Turn 7 are the first set of adds who can overwhelm you if the first 2 aren't killed quick enough. The Prosector is made trivial through use of LB3. Melusine does have an enrage but it's extremely lenient and very few people will ever see it.

Turn 8 is a lot more of a gear check, but it's also a massive test of your DPS's ability to perform at their jobs properly. They probably need to be doing 330+ to beat the enrage and the vast majority of DPS in DF/PF parties I see simply couldn't manage half that. Many will need to heavily overgear it AND require echo, so will have to wait til 2.4. It does have mechanics but they're fairly simple in comparison to the other 3 turns.

Turn 9 basically brings everything together and tests you at everything.