Steelix

This Steelix is a great Mega-Steelix and a very viable mega for Stall!

But why is the nature Careful?

As you can see, the base stat for Defense for Mega-Steelix is boosted 30 when it evolves, bringing the effective stat to 465 with no EV and IV invested.

Any more investment into Defense is overkill because when calculating damage, Attack is divided by Defense rather than Defense being subtracted from Attack.

https://bulbapedia.bulbagarden.net/wiki/Damage

A thought experiment in substituting two different Defense values for damage calculation will show that the damage will differ by only a few HP.

Thus, investing a boosted nature into Special Defense and EV for Special Defense makes Mega-Steelix a mixed tank and a better pivot overall.

In Stall or Semi-Stall, Steelix is essentially a top 3 Mega.