Here are my tips for the Tower of Dread! (For the 2ds version of Miitopia)
1. HAVE A MIXED TEAM. It may be tempting to plonk a whole team of heavy hitters in a melee battle, but bring along some healers and mixed-skill fighters as well. When I challenged the Tower, I used a team of Imp, Cleric, Vampire, and Mage, but other combinations will work as well.
2. MAKE SURE YOU'VE LEVELLED UP. A LOT. Since the Tower of Dread is an escalation battle, lower-level Miis will struggle to fight bosses like the Dark Sun and King of Hounds. I'd recommend Miis of at least Level 40 or above; my main Mii was Level 47 when I first beat the Tower.
3. SAVE YOUR SPRINKLES. Your sprinkles, especially your Life Sprinkle, are of a limited number. You should only really use them if your healer is down, or the Mii that can revive others is down. Self-sustaining Miis, such as Imp or Flower, need less input when it comes to healing anyway. Plus, there's still such thing as HP Bananas!!
4. SAFE SPOT STRATEGY. The Safe Spot is there for you when you've ran out of Sprinkles, or when your Mii is affected with an incurable Status Condition. Healing takes up an entire turn, and only 1 Mii can go in the Safe Spot at a time. However, it also enables the Mii to avoid any attack while they are sat in the Spot. Use it when you need to- especially in later fights!
5. BRING SOMEONE FROM THE HUB. The Traveller's Hub is a place where you can accept quests from people, with the prospect of a big reward. Some of these NPCs will accompany you on your journey, meaning you'll have a 5-Mii party. The higher level the NPC, the more power you'll have. Plus, Monsters don't target NPCs, which is an advantage. The only downfall is that, when your party has fallen, the NPC will not continue to battle for you. The benefits far outweigh the negatives, however!
Hope you find these tips helpful!