To reach and get past stage 30 is not an easy task. You will definitely need some legendary dice because unique dice is just not powerful enough to deal with the boss that randomize your dice even if you upgrade them to the max.
But first, if you haven't been able to get to stage 30, here is some useful dice that can get you there.
This is Iron Dice (common level), so it's easy to get. It's powerful in early game because the ATK is really high. I think without any upgrade, it deals 100 damage per hit. So having this when you just start playing this game is definitely useful.
This Mimic Dice (unique level) is useful where you can combine with any other dice for upgrade. When you combine it with Iron Dice, it will level Iron Dice to 2 dots so it will throw out 2 attack at once.
Gear Dice and Switch Dice, both of them are unique dice, should be used together because you need the flexibility to move the dice around so they are connected with each other to increase the ATK. If you pair this with Mimic Dice, you can focus on adding ATK to only Gear Dice and you won't need other attacking dice anymore. This kind of pairing will be able to bring you to stage 30 easily.
So with 4 dice above, you should be able to get to stage 30 easily. If you want to switch out the Iron Dice with some other support dice, it's good too but keeping Iron Dice somehow make it easy for stage 1 to 10. So you might want to keep it if you are pairing with beginner who don't have even Iron or Gear dice.
Since you can bring 5 dice, I suggest taking an Ice Dice to slow down the movement of enemy which increase your chance to success.
Now, to break through stage 30, you will need a few Legendary Dice.
Most important one is this, Combo Dice. This dice combo increase when it is merged. Therefore, it's important to bring Mimic Dice with Combo Dice so you have higher chance to merge it. See the image below for a more detailed explanation
I only have 1 Combo dice with 1 dot, so I cannot merge it with anything but since I have Mimic dice, I can merge it with Mimic dice to increase the combo number on the dice. The combo number I have on the image is 11. *A warning, Combo Dice ATK is quite shitty at 1 dot and 1 combo. So if you are facing a situation where you play with some player that doesn't have good ATK dice at early game, you might lose before stage 10.
This is another helpful legendary support dice, Joker Dice. It will transform into any dice it is merged with, so in this case, if you have Combo dice, you can merge Joker Dice with Combo dice and the Joker dice will transform to Combo dice so you have more Combo dice to merge with each other to increase the combo
Summoner Dice, also a legendary dice, can summon a random dice of your deck when it is merged. So if you bring this with Mimic Dice, you can merge them together to summon something. Bringing this along in your deck is definitely helpful because sometimes when you "summon" dice randomly with your SP, it will spawn too much of Summoner Dice, then you can just merge it with each other hoping to summon some Combo Dice, or any other useful dice from your deck.
Growth Dice is a bonus if you have it. It will automatic grow into a random dice with extra dot after 15 seconds. It might help you to reach max dots (7 dots but it will be shown as a star) very early in the game but you will need some luck to get the best dice. If you end at useless dice with maxed dots, then you can only hope the randomiser boss will change it to something good, or Leon to downgrade your dice so you can change it to something else (with Mimic or Joker dice)
This is not a bad dice too but not so useful in late game. I will say if you have this dice, but you don't have any other unique dice like Gear or Switch Dice, you can use this to get to stage 30 easily. It will drop a nuclear bomb on the first monster it hit and everything around it with a small radius when it is merged. So bringing Mimic Dice with this can make you have a lot of safe play. If you have ice dice that can make them slow down and gather together, dropping this bomb will be even more effective.
You can watch the video of how I get through stage 30. But some bad luck when my maxed dots dice got randomised into Mimic Dice that doesn't deal enough ATK to get me to later stage.