How do I use Gladiator Beasts and how should I build them?

I'm not even going to lie, it's just when I tried making Gladiator Beasts and looking them up and such, I never really understood their success, like with the whole tagging/fuse thing for the win. By everyone's response they seem really good, and I got beat by others that used them to, can anyone help introduce me to Gladiator Beasts?