Me and my buddy always struggle with this one:
Arby sets off to your base the moment the temple is complete. Forge just goes for supplies and funds him to upgrade/purchase suicide grunts.
We manage to see off the rush nine times out of ten, but the arby preoccupies us and causes damage for long enough for Forge to build up an insanely strong 2-base economy and pour huge amounts of upgraded scorpions at us.
This seems to be one of those bog-standard tactics that we come up across all the time, but we are really struggling to come up with an answer; even when we know exactly what they are going to do.
Have a brute.
Send brute to arbys base, forcing him to defend.
Option 1 : He defends, you get free summits.
Option 2: He doesn’t, gets massively out damaged, you win.
In combination with the above strat:
Have a UNSC.
Send hogs to arbys base, ram + gunner down his suicide grunts as they try to go through lift.
Option 1: He recalls, you split and lose a hog or two.
Option 2: You gunner his pads till he has none, then go kill him.
Proceed to build banshees/vamps/chief/gauss/tank/wolve and win while arbiter is forever tech 1.
As a good arby you can play defensively and come out ahead. As a POR you can play similarly to a brute but keep your units on D if they have no gunner.