Thanks for your reply, and your advice is helpful!
First, I think invest more def for Tangrowth aginst Swampert is necessary and I will do that latter.
As for the scarf garchomp, the main reason I choose it is I hope it works as a counter for the boosting move uesers, such mega Charizard-X, Volcarona, Dragonite. etc.
It can outspeed them after +1 rank of speed, and kill them.
Therefore, my metagross choose a larger coverage rather than a priority move.
What you think about this probelm? how to tackle the issue of boosting move uesers? For example, the situation is that my Tangrowth now is facing Volcarona, and I know it will Quiver Dance next turn. Of course, Azumarill with aua jet can deal with this, but how about a Charizard-X with dragon dance?
Look forward for your opinion