Combine is the only option to advance your Warrior to the next Form of evolution, which is the only way to max out the stats for each of your warriors.
You can only use the "Combine" option on Warriors that have reached the max level of their current Form.
Combine is made by using other Warriors on a required Form (no max level is needed for combine materials) to evolve a Max Level Gotcha Warrior, additional energy is required to finish the process.
Do not worry, something that differences this game from others is the fact that all evolutions and mixes are 100% guaranteed all the time.
Not all Warriors can be Combined, and not all of the ones that can have the same number of Forms (some can evolve more times than others), one example is "Itachimaru", which comes at Max Level when you get it and can't be evolved (it's used as a Combine Material, though).
|G.W. TO EVOLVE.||G.W. NEEDED.|
|BILLY THE HUMMER|