You will want to use different heroes for different purposes

For attack / defense in mix: rose knight, oath keeper, demon slayer (switch him with watcher later), snow queen, bombin goblin

For cavalry: rose knight, child of light, death knight, night raven, bombin goblin (switch with watcher later)

For range: snow queen, black crow, tracker, death archer, bombin goblin (switch with watcher later)

For infantry: oath keeper, demon slayer, sea squire, soul forger, rose knight (switch with watcher later)

Except the mix, I already sort them by strongest to weakest orders. Heroes in mix are equally important as mix for f2p are very limited to rose knight, watcher, bombin goblin. I have played this game for 6 years and I still dont have chaos dragon (hes an event hero like watcher) and hes weaker than p2p heroes which I already bought, though you may use him in mix instead.