The worst encounter would still be Gruul. The design of it is just wrong, completely wrong. Now that it's nerfed, it's not that hard anymore, but when it was released it was pure horror.

我认为设计的最差的boss是格鲁尔，设计完全错了，现在他已经被nerf了不再那么难，但是在最初相当难。

youdao

Like Gruul, Boros is a guild that is forced into the attack role in order to reap the benefits of its guild mechanic.

FORBES: Gatecrash: Evaluating Boros

Dimir is much more subtle than Boros, Gruul, or Simic, but stabilization and multiple connects with an encoded spell can usher in disaster.

FORBES: Gatecrash: Evaluating Dimir