There are a few challenges with the waves in this dungeon:
Let's address them in order:
You'll need to play with your AI settings a good bit to fine tune your waves. The first thing you need to be sure of is that all of your champs arrive at the boss with their abilities ready to use. This usually means for Wave 2 you want to open-only, then don't use their important skills. The hope is that you can get those abilities back up before clearing the wave and be ready for the boss.
Note that for especially fast wave clears- you might not even be able to use certain abilities as wave 2 openers or even in wave 1. For example, one of the Cardiel A3s has to be disabled in wave 1 to ensure it's up for the boss in some speed record runs.
If you have to choose, save Gnuts A2 for the second wave. It is a much harder/slower wave, and his debuffs and damage can do a lot to soften that wave up a bit.
Def down on the waves helps, as does Crushing Rend. If you're only doing 1-2 skill rotations on each wave, Crushing Rend less than 6* is still incredibly powerful for its defense mitigation.
Accept it's going to happen, or try the longbeard variant if you're doing Double-AA. It may work in your favor if cardiel is built do do a lot of damage, but it also slows down a run since it is a single target attack and may be in the way of more damaging AoEs from the rest of your team.
Genzin has some of the fastest animations, so he naturally works really well for speeding up wave clears. On the other hand, dwarves (like our Gnut friend) tend to have a much longer animation as they waddle their way to the ever patient boss. If you have options, choose the ones that have faster animations to save yourself time on the waves.
Riho. She's the ultimate end to your would-be fastest wave clear ever. She sluffs off the first debuffs you land on her, is fairly tonky, and she hits like a freight train carrying the disappointment of the Raid community at trans-continental speeds. She will fuck up your run.
There's a few ways to deal with her:
Riho will target your champion with the lowest hp. You can HP tune who she targets and make it someone who has a combination of high def and/or hp.
If you can spare it (especially in Double-AA teams), you can use Cardiel's A2 as an insurance measure. Have him open A1 and prioritize A2 after that. With luck, if Riho nukes someone, they'll get a revive and it won't be the end of your run.
Burn her debuff passive. If you can land a non-critical debuff on her first, and then follow up with your defense down/weaken debuffs, she'll go down a lot easier.
Violence is always an answer. Kill her before she kills you.