Achievements sightings, are some more rare than others?

Off Topic
1

What is the rarest achievement you have seen?
For me it was the Bad Beats Green Cat from the special category of Achievements 1.0

2

I agree. That is a very rare badge to see.

3

I was told there is one achievement that can only be won in the team championship game.

4

What does this mean?

And, how is it Achieved?