the Oceanborn:

First the Prophet of Regret was overpowered, now the Arbiter is too? Good gods, people. Could it just be that the Asian dude was a better gamer than you? Knew how to stock up supplies and head straight for the throat?

The Covenant leaders are fine. Using a quick reactive strategy, you can take them down. They're not overpowered.

i just think that you didnt expect him to do that, you just didnt preapre in the right way, the covies have an advantage off triking harder - faster than the unsc that strike even harder-slower. 

 i suggest you get a bit of infantry going to fuel an army, then you bring out some scorpions and hornets.