User blog comment:Sylvandyr/The math behind the Battle Arena/@comment-5089372-20130611171107/@comment-4830404-20130613221420

Yea I'm not sure what increases their level, but I feel like the first few battles had low-ish levels, then after that I've always faced dragons above level 10.

Diamond (provided you have one) does very well against other Diamond types. Since the Diamond type has a strength against Diamond and no weakness, this means that your Diamond type will get a bonus, but the enemy's type being Diamond will not punish yours. So, for me, Diamond is the "best" matchup against any Diamond types unless I have a Diamond hybrid that has an additional strength against an enemy Diamond hybrid.