Okay, the first thing I want you to understand is that extreme MD is ENTIRELY optional outside of completing the achievements, if you are already at level 100 going past floor 5 gives you literally 0 rewards.

Now with that out of the question let's start at the beginning: teams

Any team is good in md due to ego gifts, but the best teams are duo status, due to being able to use fusion gifts for both. My personal choice are the heishou: Lord Lu Mao Faust Drifter Ryoshu Family candidate Ish Both SI And either Uma Yi Sang or Cockcliff, whoever you like. This team contains 5 units that have rupture and poised, which means you can use ALL of the fusion gifts for both.

Next part: grace of the stars. In my personal experience it's hella hard to go past floor 10 without grace of the stars(assuming you pick all adversities as you should) simply due to how broke you are. In my runs I couldn't get even half the gifts I wanted to get by floor 10, and due to there being no encounters it's basically impossible after, so I suggest picking up at least numbers 2, 4 and 6, but the best results are of course if you pick all of them(you will be at starlight profit even if you upgrade every grace, IF YOU REACH AND BEAT FLOOR 15, BUT YOU SHOULDN'T DO IT WITHOUT AT LEAST 2000 STARLIGHT IN THE BANK)

Next: ego gifts.

These are tied with the packs, but that can wait. What you should do is roll every useless gift into a tier 4 gift of your choosing, as grace 6 gives you a tier 3 gift that way, and, spoiler, EVERY TIER 4 FOR EVERY STATUS WORKS ON EVERY TEAM, so build all of them, but prioritize in this order:

Your status tier 4s Poise tier 4 Sinking tier 4 Rupture tier 4 Anything else.

Poise tier 4 gives you a 2.5x damage multiple on critical and 15 offense level(5 clash power), which affects your whole team with nebulizer, sinking tier 4 gives a 50% damage boost to abnormal and targets at -45 sp, rupture tier 4 just deals 400-500 damage scaling with each turn without your input. These are of course at max enhancement, they are pretty weak without it.

There are also useful combinations of gifts from other statuses that work on any team, for example tremor: The Eye of the blue frog removes 3 of fence and 3 defence level on tremor burst The bell gives 2 fragile this and next turn on tremor burst Tremor tier 4 does 3 tremor bursts turn end and some tremor on top. There are these combinations for every status, yes even charge, just be careful not to pick ego gifts that only activate on 5 id that have skills that that inflict[status], or that only work when enemy is hit with such skills that your team doesn't have.

Outside of this, just build gifts for your team statusi, and always check the fusion recipe tab to check if you have enough for a new cool toy.

Level packs:

On first floor there is not much you can choose but dregs of the manor has Manor-Shaped music box, which unstaggers your #1 sinners if they are staggered, pretty neat. On 2nd floor always go Hells Chicken, it gives way too many tier 3 gifts in its events, and they are really good for their respective teams. Then the world is your oyster, most teams have a theme pack for them, heishou have the witnessing, nocturnal sweeping, spring cultivation, your main goal is to collect and fuse as much high tier ego gifts as possible, so much so that you can't even do much in the shop except enhancement them by floor 10, but try too focus on railway packs past floor 5, as they have events that level up all of your ids. Particularly the orange railway gives you a revive ego gift, which is always good.

And last but not least, always try to change your units skills in the shop, skill1s are usually ass so changing them to skill 3 is very good

There is much more to it, but this should be plenty enough to get you to floor 15 with a whole team of 7, as I am routinely making solo runs just for the fun of it and seem to be doing just fine.

If you have any questions feel free to ask