Your party can only contain 5 heroes at a time. So eventually you have to choose which heroes you want to use, and which ones you don’t. There are researches and upgrades that only boost a certain type of heroes. You want to limit the amount of researches and upgrades you need to level up, yet boost the most heroes while doing it. Currently the best strategy is focusing on a mana spellcaster hero party. Out of the first 8 heroes, 4 of them uses mana and they are also spell casters. So eventually you get a party that contains Solaine, Benedictus, Blaze and Luana. Also considering that Solaine and Blaze are the most favorable leaders in the game, I don’t see this setup change anytime soon.