Got some practice in with my Bulky Four Attacks set tonight, and the results haven't really been that impressive. The first thing I noticed was just how difficult it is to actually use him in play, because Fire and Ice are EXTREMELY common attacking types right now, and even Flying is very prevalent due to Brave Bird Blazikens and Talonflames. Even with his Assault Vest, he still takes a large enough amount of damage from Greninja's Ice Beam to the point where it will 2HKO him if he doesn't get the opportunity to Horn Leech in between attacks, so you can't really switch him into an incoming attack that he's weak to. (If he does land the Horn Leech inbetween, however, he'll score an OHKO and be back at 85% HP minimum haha).
Horn Leech is a lot tougher to use reliably than I first thought as well. Fire, Flying, Bug, Grass, Dragon, Poison, and Steel all resist Grass-type attacks. While Trevenant does have great coverage through his other moves, he is pretty reliant on healing through Horn Leech because even with his bulk, he still has trouble taking a lot of punishment without healing inbetween.
I definitely need to do more research with this set, but as far as my research has shown, it's more of a niche set that's really only optimized for patching up teams that have weaknesses to Water or Ground 'Mons.
EDIT: After more testing, I have concluded that Bulky Four Attacks Trevenant is more of a liability than an asset and should not be used in competitive play. He's way too niche and frankly just sucks.