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...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.
Eviolite applies its boost after most stat modifications, so presumably it would simply apply the boost after the changes from Guard Split. In other words, if you had a Pokemon with an Eviolite and 100 Defense, and you used Guard Split from a Pokemon with 200 Defense, the first Pokemon would have the equivalent of 225 Defense. [(100 + 200)/2*1.5]Only issue I have with Chansey tbh is that it has even less offensive presence than Guzzlord. Guzzlord can at least put out some damage and actually become a threat if you run Weakness Policy, or if you want combine it with Tapu Fini for Swagger and Heal Pulse support. Chansey just doesn't have that, it has to rely on Toxic or Seismic Toss to actually do anything to opponents. But other than that, you're right. In terms of bulk Chansey is superior if you're Guard Splitting, although I'm not sure actually how Guard Split works with Eviolite. I don't think it takes the Eviolite into account, but I'm not entirely sure.
It's a Tank.There is something I never really understood about Guzzlord. What do you think GameFreak intended it's playstyle to be? It's bulk is a coin-toss, it's attack is a coin-toss. I like the high HP stat and it does reflect Guzzlord well but I feel like more could be done with this Pokemon considering it barely has any moves to support that high HP stat. An odd Pokemon don't you think?