its better to keep leveling up and unlocking more characters(hopfully higher tier ones because they can use more skills) the more characters you use on developers mode the more multipliers and bonuses you unlock

Rolling for characters at the store gives you the chance of earning cards of higher rarity from C to R to S to U.

R characters gain more stats from level up than common characters, S and U characters also gain more talent fragments from talent squares so you can get more powerful skills and more xp from growth squares so you can level up higher than you were able to before.

Additionally, progressing further in the tower unlocks scrolls which allow you to skip the early parts of development mode letting you get to the important islands (3 and 5) much faster so your cards get both more levels and more powerful skills again letting you progress further in the tower and so on.

If you get stuck at a section try looking at unique challenges in the Usami flower in order to get gold or pink monocoins to buff your characters with W cards or get new U cards to add to your current team.

U cards are the best thing in the game I already have 4 U cards of my favourite characters also they can have 5 skills but there are 2 skills you will need on all cards to help you in the monokuma tower (in my opinion) there are “Ultra Defense & Endurance Up”, there are the best skills you will need like “Ultra Defence” can protect from powerful attacks and “Endurance Up” can protect you from bad status effects, I think both skills are very important to take on monokuma tower also you will need to level up them up as high as you can.