Anime Squadron progression is built around a simple loop: summon units, clear Story stages, collect materials, craft gear, then improve traits and stat ranks.
Early on, do not chase Secret units before you have a usable team. Cost, cooldown and range matter as much as raw damage because stable clearing gets you more rewards.
Use the unit wiki to compare base DPS and cost. Then use the rewards table to choose stages that give Gems, XP and materials at the same time.
Useful next pages: /games/anime-squadron/characters, /games/anime-squadron/items, /games/anime-squadron/stages.