To get to rank 5, you just have to kill Thrall again.
Rank 6 requires an additional objective.
Also, the reward system can be a bit confusing. Here is the blue post on it:
To get to rank 5, you just have to kill Thrall again.
Rank 6 requires an additional objective.
Also, the reward system can be a bit confusing. Here is the blue post on it: