My offensive experiences with Terrakion:
Besides checking Ekiller, Stab Stone Edge theoretically slays Ho-Oh. However, while in OU Scarfrakion can switch into a +1 Volcarona without taking damage from its stabs, Ho-Oh is a different matter.
If you somehow avoid the burn from Sacred Fire, and can stomach the 43.2 - 50.92% sun-boosted hit, Ho-Oh can just switch out (and is actually rewarded for doing so if SR is not up). Brave Bird also deals a massive chunk (68.82 - 81.48%). Therefore, it is much easier to revenge Ho-Oh than to come in and actively check it.
His biggest problem:
Landing Stone Edge Choosing between Stab Moves that will hurt one enemy but not the other (IE Lugia is out with a Gira-O in the wings).
Solution: Toxic as the fourth move. Though unorthodox, this functions as a great way to preserve your momentum and wallbreak. Additionally, SD or CM Ghostceus (really any non rest or refresh late game Arceus that is not heavily damaged by his STABS or X-Scissor) can now be put on a timer.
I am not just theorymonning (I have replays but cannot easily find them), it has worked very well for me in the past and is worth a shot to run Toxic on Terrakion.
I would definitely like to hear other experiences with using boosting Terrakion, as otherwise his power can be underwhelming.
Now an EMT for Los Angeles county. PM me for house calls. (Totally legit I swear.)
PM me for an OU team rate!