One of my alliance fellows is F2P and started Empires at the beginning. He just got his first 5* Hero from his legendary training. Getting training camp 20 built and unlocked is something I made a priority.
Since 5* Heroes need the same ascension items as two 4* Heroes, I am leveling up all my 5* Heroes to 2.60 and then stopping, until I have three 4* Heroes of every color. Not only does that give my team more flexibility, you need different heroes for farming, rare quests, titans, raiding, war and challenge events.
But you can dramatically increase your damage by attacking a single boss with two heroes of a strong color and removing your weak color hero. Example: my green 4* healer is weak against red titans and bosses, I can swap in a blue 4* hero ( strong against red ) and my damage increases because I removed a hero doing half damage, and added a hero that does double damage.
If I don’t have a good blue 4* hero, I will use a maxed out blue 3* hero with a high attack stat. The 3* will die quickly, but will still give the increased tile damage from a second blue hero ( strong against a red titan or boss )
For challenge events it helps to remove the reflected color, and the weak color, for single bosses, and have three heroes of the strong color.
If you cannot get ascension items, raise your 4* heroes to 3.60 and your 3* heroes to 3.50, they will still help you out a lot if you can swap in and out the right colors and special skills.