Here's a fun Buzzwole set I've messed around with.
Vodka Tea (Buzzwole) @ Leftovers
Ability: Beast Boost
EVs: 200 HP / 64 Atk / 184 Def / 60 SpD
Adamant Nature
- Substitute
- Ice Punch
- Focus Punch
- Leech Life
I like the idea, sort of a bulkier take on the standard SubPunch set. However, your EVs could use some optimizing.
For example, you mention that your HP and SpD EVs give Quagsire only a 25% chance to break your Sub, but I don't think that's true. Here's the full damage range for Quagsire's Scald:
0 SpA Quagsire Scald vs. 200 HP / 60 SpD Buzzwole: 93-109 (22.9 - 26.9%) -- 38.3% chance to 4HKO
(93, 93, 94, 96, 96, 97, 99, 100, 100, 102, 103, 105, 105, 106, 108, 109)
200 HP EVs gives you 101 HP subs, which means there's actually a 7/16 chance for it to break your Sub. Even besides that, since you have decided to just go for more bulk instead of investing more in Spe and Atk, you'll benefit more by investing in HP first. For example, if you maximize his HP by moving 16 SpD EVs and 36 Def EVs to HP, you actually reduce Quagsire's chance of breaking your Sub slightly while also slightly boosting your bulk on both sides. Here's some calculations to demonstrate.
0 SpA Quagsire Scald vs. 252 HP / 44 SpD Buzzwole: 93-111 (22.2 - 26.5%) -- 16.8% chance to 4HKO
(93, 94, 96, 97, 97, 99, 100, 102, 102, 103, 105, 106, 106, 108, 109, 111)
Max HP Buzzwole makes 104 HP Subs, so you can see from the damage range that Quagsire now only breaks your Sub 6/16 of the time.
252 SpA Heatran Lava Plume vs. 200 HP / 60 SpD Buzzwole: 392-464 (96.7 - 114.5%) -- 81.3% chance to OHKO
252 SpA Heatran Lava Plume vs. 252 HP / 44 SpD Buzzwole: 404-476 (96.6 - 113.8%) -- 75% chance to OHKO
+2 252 Atk Terrakion Continental Crush (180 BP) vs. 200 HP / 184 Def Buzzwole: 382-451 (94.3 - 111.3%) -- 68.8% chance to OHKO
+2 252 Atk Terrakion Continental Crush (180 BP) vs. 252 HP / 148 Def Buzzwole: 393-463 (94 - 110.7%) -- 62.5% chance to OHKO
Just a couple of random examples to demonstrate the slight reduction in damage and KO chance. It's not a huge difference, but it's still an optimization all the same. There's probably a better benchmark for Def and SpD that you could aim for (you'll need 252 HP / 92 SpD to completely prevent 0 SpA Quagsire from breaking your Sub, for example), but this is just a little change to help improve what you have so far. Hope it helps!