How do you execute combo moves because I haven't completed one of any best..

This can be Hard for beginners, but Easy to answer:
When you're killing enemies, you'll notice a "" icon in the bottom right hand corner of the Screen. This is your "Combo", and it will slowly drain if you don't successfully defeat an enemy.

So basically, with every enemy you successfully defeat, your combo will increase, but if you take too long defeating an enemy, your combo will be broken.

An easy way to ensure your combo is never broken is to fill up your Awakening Gauge then Activate it. When awakened, every enemy you attack with Circle in succession will die in one Hit.(except for those with a large HP Bar) I did that on "The Octopus and the Shark" Stage, and I was able to reach over 100-200 hit combo. But that was only because the enemies were endless.

An easy way to fill up your awakening Gauge is to equip the following Cards:
Guy
The Uchiha Clan
Kurenai Smiles
The Nine-Tails Before
Fukasaku + Shima
Grown Nagato + Grown Konan + Grown Yahiko

There are other Cards that raise your awakening, but they require Certain conditions such as health being below 50%, During Team Fight Bonus only, etc.

Hope this helps!