One thing I've tried with Guzzlord is to use a Guard Split Shuckle to boost its bulk. After the Guard Split Guzzlord gains effectively base 223/150/150-ish defences, which is insanely bulky when it also has Stockpile to boost further, and stage boosts stack by multiplication with the Guard Split (as in, each Stockpile adds 50% of the post-Guard Split defence stats, not the original defence stats). It becomes so bulky even fairy hits have a hard time 2KOing it. But its damage output is poor and it has literally no way of boosting that damage output aside from the situational Weakness Policy (Beast Boost starts boosting a defensive stat after Guard Split), making it prone to eventually getting worn down if not simply ignored by the opponents simply focusing on Guzzlord's partner. Simply put, the bulk becomes otherworldly insane, but it doesn't have enough direct threat to the opposition to turn that bulk into a win condition in a double battle. The exact calculations depend a bit on how you invest Guzzlord and Shuckle, but here's an example just to get a rough idea of what it can take:
252 Sp.Atk Tapu Lele Moonblast vs. +1 252 HP / 0 Sp.Def Guzzlord after Shuckle Guard Split: 156-184 (47.2 - 55.7%) -- 64.8% chance to 2HKO
That's STAB, 4x super effective, coming from 130 base special attack, and it doesn't even guarantee the 2KO. Brave Guzzlord can OHKO Tapu Lele back with Heavy Slam. Compare the bulk on a (very heavy) neutral hit:
252+ Sp.Atk Adaptability Porygon-Z Hyper Beam vs. 252 HP / 4 Sp.Def Eviolite Chansey: 124-146 (34.7 - 40.8%) -- guaranteed 3HKO
252+ Sp.Atk Adaptability Porygon-Z Hyper Beam vs. +1 252 HP / 0 Sp.Def Guzzlord after Shuckle Guard Split: 92-110 (27.8 - 33.3%) -- 1 in 4096 chance to 3HKO
After a single Stockpile, it's bulkier on the special side than Eviolite Chansey, and it's just as bulky on the physical side. Anything that isn't fairy damage, or a guaranteed portion of health taken out (like Toxic), isn't going to hurt this monster if you manage to set it up.
This is nice and all, and Guzzlord may be bulkier than a base Chansey after Guard Split, but if you were to have Guard Split the Chansey instead...
252 SpA Adaptability Porygon-Z Hyper Beam vs. 248 HP / 8 SpD Eviolite Chansey after Guard Split from Shuckle: 132-156 (18.7 - 22.1%) -- possible 5HKO
Assuming 252 Sp.Def / 252 Def Bold Shuckle, Chansey effectively has 250/117/182 bulk after Guard Split... with an Eviolite. And recovery. This means that even in a double battle where it might be taking attacks from two opponents at once, it's still going to be extremely difficult to bring down compared to Guzzlord, whose only means of recovery is Rest.
Not that I'm condoning this strategy in the first place, it seems very gimmicky and requires a lot of set-up for a fairly small payoff, but if you're going to Guard Split something, Chansey is the perfect choice for mostly the same reasons that you'd consider Guzzlord on paper (high HP, low to mediocre defenses), but it also has recovery and Eviolite to make it even harder to take down. Then again, this thread is about Guzzlord, not Chansey.
Anyways, Guzzlord gets the questionable honor of being the only Ultra Beast to drop all the way to the lowest tiers. Looking at it, it's unfortunately quite clear why this is the case. Guzzlord has an extremely strange stat spread, which is characteristic of all Ultra Beasts, but unfortunately affects Guzzlord the worst. On paper its extremely high HP (third only to Chansey and Blissey) might seem like it gives it a niche as a solid wall or tank, but in reality that sky-high HP stat is undermined by awful 53/53 defensive stats, a lack of recovery options, and a very poor defensive typing, having weaknesses to Bug, Fighting, Dragon, and a crippling 4x weakness to Fairy. Its offensive stats are decent but fail to stand out in any way, at 101/97.
I haven't used Guzzlord since the beginning of SM, but the one set I did use that could occasionally put in work was a Choice Specs set. The problem is, with Choice Specs it's basically a worse Hydreigon, as it lacks Hydreigon's offensive stats and ability to at least outpace most defensive Pokemon. If you really want to use Guzzlord in singles however, it's probably about your best bet.
Guzzlord @
Choice Specs
Modest - Beast Boost
248 HP, 252 Sp. Atk, 8 Sp. Def
-Draco Meteor
-Dark Pulse
-Sludge Bomb
-Flamethrower
Pretty straightforward set. Draco Meteor and Dark Pulse are your STABs, boasting great power at the cost of lowering your Sp. Atk, and decent power with no drawbacks respectively. Sludge Bomb nails incoming Fairies who wall your STABs, and Flamethrower at least does something to Ferrothorn, Magearna, etc. The problem is, you can run almost this exact same set (with Flash Cannon instead of Sludge Bomb) on Hydreigon and put in way more work in most games, or you can run a more conventional wallbreaker entirely, such as Kyurem-Black, Tapu Lele or Mega Medicham. There just isn't really any reason to use Guzzlord in inclusive formats.
I kind of wish GameFreak had designed him as a Dragon/Steel type and perhaps given him some kind of recovery move. That might have given him a more solid niche.