EDIT: Roar, new page again!
You can't just add up the flat percentages and expect to get the right answer. Just for everyone's reference, and somewhat to show why Substitute really isn't all that great on Garchomp, let's take a standard SubSalac Garchomp and assume a 4HKO (Three Subs, SR chip damage preventing a fourth, plus the KO.)
Chance of Sand Veil causing a miss through four (100%) attacks: Oh, I see ultimifier already did the "math lesson" (over five attacks) and even Hip checked his own. Whatever, I already typed this up. :P
0 miss: 40.96%
1 miss: 40.96%
2 miss: 15.36%
3 miss: 2.56%
4 miss: 0.16%
Actually, since an earlier miss saves you from having to Sub as often, it's probably a good idea to note the cumulative chance of hitting x times in a row as well, which is simply 80%^x. Anyway, the chance of coming out with a fresh Sub over three turns is only 1-.8³ (48.8%), or less than half the time. In other words, if you spam Sub trying to force a Sand Veil miss against anything that can break a Sub in one hit (i.e. most pokemon), you're just gonna end up 75% health in the hole over half the time and you'll still be in revenge-kill range for Scizor 80% of the time. :P
Substitute is still an okay move for easing prediction (Oops, Earthquaked that Latias switch-in! Crap, Outraged on Scizor!) but it's not really an effective way to abuse Sand Veil. Yeah okay, a couple of early misses and you're set up +2 behind a Sub. Well you know what? You could've just done without the Sub, those moves would've missed you anyway and now you have an extra moveslot to work with. The only advantage to Sub at that point is that it protects from revenge-killers that can OHKO, like Latias (Meteor) or Starmie (Ice Beam).
It's not like Garchomp doesn't appreciate the extra moveslot. Carrying Dragon Claw as its only Dragon move takes away a lot of the power which makes Garchomp most scary. Likewise, carrying only Outrage makes it more susceptible to being revenge killed, as Garchomp is forced to lock itself in against most Flyers/Levitators to nab a kill. Sub/SD Garchomp usually only carries its STAB moves, wouldn't it appreciate having Fire Fang so half the OU Steels don't counter it?
Of course my example is Garchomp switching in. The entire point of my "example" is to illustrate what commonly happens when a Garchomp is switched in. But the opposite is also a very common scenario I see myself in when facing Garchomp, my switching Swampert into it (scaring off my support Jirachi) as it tries to set up. I kill the Sub, eat a +2 EQ for 90% and kill it. Against the non-Sub ones that SD off the bat, that's usually a ~70% Ice Beam through Yache Berry and Swampert dying, then I just clean it up with Scizor or Latias. If Sand Veil screws me over once, I have the other one of those two as backup.
Am I overprepared for Garchomp? I wouldn't say so. My primary switch-in and "crippler" is [generic bulky pokémon with Ice Beam] and I have two backup checks in Scizor plus [generic offensive pokémon that outspeeds Garchomp]. Everyone else is already using Scizor anyway, while any number of bulky pokémon could replace Swampert (e.g. Cresselia, Suicune) and faster pokémon replace Latias (e.g. Azelf, Starmie, Scarfers). Other priority users can replace Scizor or work alongside it to deter Garchomp further. My team also lacks a Reflect user and uses neither Spikes nor Toxic Spikes, all of which can greatly help anyone beat Garchomp without losing half a team. I also have neither Skarmory nor Bronzong, the two accepted "counters" to Sub/SD sets and still two of Garchomp's best checks otherwise. No bulky Gyarados. So... overprepared? Not even close, yet I have no trouble handling Garchomp! Overpowered? It's not even as dangerous as Latias or as metagame-defining as Tyranitar. (Sand Stream...) If Garchomp was that bad, I'd be using Ice Punch or Reflect on my Jirachi for it, instead of Tbolt for Skarm and Gyarados. :P
If Sand Veil screws me over three times or whatever then yeah, I lose, but that just doesn't happen often enough for me to consider it broken. Hell, I got 6-0'ed by a Gyarados earlier today because Ice Fang flinched me at a very inopportune moment and I'm sure plenty of other people get into trouble with Gyarados as well when Waterfall does it. Probably half of my losses have been to CM Latias variants just because I'm not using T-Tar to counter it and lack a Scarf Outrage user to revenge. Garchomp isn't the only pokémon that sweeps teams with "middling" or "high" levels of hax and it's not the only major offensive threat people have to prepare for.
And hey, Garchomp is susceptible to a lot of the same crap many other threats are. Reflect works against a lot more than Garchomp. In fact, it's primarily used on Rotom and Psychics to deter Pursuit users from ruining their day, not because of Garchomp. Ice moves work on more than just Garchomp, they also hurt them other SCARY DRAGONS (except broken Kingdra) and have generally good type coverage. Spikes wear down T-Tar and Vaporeon and Toxic Spikes ruins them just as well as they do Garchomp. Jolteon outspeeds and Skarmory walls more pokémon than just Garchomp. Priority moves help against... anything that's faster than you. Making sure your team is prepared for Garchomp isn't any different than preparing for one of 50+ other threats out there. Hell, you could even use Machamp or a weather user (e.g. Abomasnow, RD Manaphy) to stuff Sand Veil. Scarf Abomasnow itself is a great check and revenge-killer, to all three suspects at that. Granted, Abomasnow is more of a team commitment than a stand-alone weather user but then so is T-Tar/Hippowdon. :P
If it didnt dodge the first two ice beams it could have dodged any two or any one of the icebeams and the bullet punch. That is a 40% chance.
You can't just add up the flat percentages and expect to get the right answer. Just for everyone's reference, and somewhat to show why Substitute really isn't all that great on Garchomp, let's take a standard SubSalac Garchomp and assume a 4HKO (Three Subs, SR chip damage preventing a fourth, plus the KO.)
Chance of Sand Veil causing a miss through four (100%) attacks: Oh, I see ultimifier already did the "math lesson" (over five attacks) and even Hip checked his own. Whatever, I already typed this up. :P
0 miss: 40.96%
1 miss: 40.96%
2 miss: 15.36%
3 miss: 2.56%
4 miss: 0.16%
Actually, since an earlier miss saves you from having to Sub as often, it's probably a good idea to note the cumulative chance of hitting x times in a row as well, which is simply 80%^x. Anyway, the chance of coming out with a fresh Sub over three turns is only 1-.8³ (48.8%), or less than half the time. In other words, if you spam Sub trying to force a Sand Veil miss against anything that can break a Sub in one hit (i.e. most pokemon), you're just gonna end up 75% health in the hole over half the time and you'll still be in revenge-kill range for Scizor 80% of the time. :P
Substitute is still an okay move for easing prediction (Oops, Earthquaked that Latias switch-in! Crap, Outraged on Scizor!) but it's not really an effective way to abuse Sand Veil. Yeah okay, a couple of early misses and you're set up +2 behind a Sub. Well you know what? You could've just done without the Sub, those moves would've missed you anyway and now you have an extra moveslot to work with. The only advantage to Sub at that point is that it protects from revenge-killers that can OHKO, like Latias (Meteor) or Starmie (Ice Beam).
It's not like Garchomp doesn't appreciate the extra moveslot. Carrying Dragon Claw as its only Dragon move takes away a lot of the power which makes Garchomp most scary. Likewise, carrying only Outrage makes it more susceptible to being revenge killed, as Garchomp is forced to lock itself in against most Flyers/Levitators to nab a kill. Sub/SD Garchomp usually only carries its STAB moves, wouldn't it appreciate having Fire Fang so half the OU Steels don't counter it?
Also your examples are assuming that Swampert is already out against Garchomp. In Haunter's example Pert switched into Chomp. It isnt hard to force a switch with Garchomp.. Especially one that is indistinguishable from scarfchomp until it uses sub or SD.
Of course my example is Garchomp switching in. The entire point of my "example" is to illustrate what commonly happens when a Garchomp is switched in. But the opposite is also a very common scenario I see myself in when facing Garchomp, my switching Swampert into it (scaring off my support Jirachi) as it tries to set up. I kill the Sub, eat a +2 EQ for 90% and kill it. Against the non-Sub ones that SD off the bat, that's usually a ~70% Ice Beam through Yache Berry and Swampert dying, then I just clean it up with Scizor or Latias. If Sand Veil screws me over once, I have the other one of those two as backup.
Am I overprepared for Garchomp? I wouldn't say so. My primary switch-in and "crippler" is [generic bulky pokémon with Ice Beam] and I have two backup checks in Scizor plus [generic offensive pokémon that outspeeds Garchomp]. Everyone else is already using Scizor anyway, while any number of bulky pokémon could replace Swampert (e.g. Cresselia, Suicune) and faster pokémon replace Latias (e.g. Azelf, Starmie, Scarfers). Other priority users can replace Scizor or work alongside it to deter Garchomp further. My team also lacks a Reflect user and uses neither Spikes nor Toxic Spikes, all of which can greatly help anyone beat Garchomp without losing half a team. I also have neither Skarmory nor Bronzong, the two accepted "counters" to Sub/SD sets and still two of Garchomp's best checks otherwise. No bulky Gyarados. So... overprepared? Not even close, yet I have no trouble handling Garchomp! Overpowered? It's not even as dangerous as Latias or as metagame-defining as Tyranitar. (Sand Stream...) If Garchomp was that bad, I'd be using Ice Punch or Reflect on my Jirachi for it, instead of Tbolt for Skarm and Gyarados. :P
If Sand Veil screws me over three times or whatever then yeah, I lose, but that just doesn't happen often enough for me to consider it broken. Hell, I got 6-0'ed by a Gyarados earlier today because Ice Fang flinched me at a very inopportune moment and I'm sure plenty of other people get into trouble with Gyarados as well when Waterfall does it. Probably half of my losses have been to CM Latias variants just because I'm not using T-Tar to counter it and lack a Scarf Outrage user to revenge. Garchomp isn't the only pokémon that sweeps teams with "middling" or "high" levels of hax and it's not the only major offensive threat people have to prepare for.
And hey, Garchomp is susceptible to a lot of the same crap many other threats are. Reflect works against a lot more than Garchomp. In fact, it's primarily used on Rotom and Psychics to deter Pursuit users from ruining their day, not because of Garchomp. Ice moves work on more than just Garchomp, they also hurt them other SCARY DRAGONS (except broken Kingdra) and have generally good type coverage. Spikes wear down T-Tar and Vaporeon and Toxic Spikes ruins them just as well as they do Garchomp. Jolteon outspeeds and Skarmory walls more pokémon than just Garchomp. Priority moves help against... anything that's faster than you. Making sure your team is prepared for Garchomp isn't any different than preparing for one of 50+ other threats out there. Hell, you could even use Machamp or a weather user (e.g. Abomasnow, RD Manaphy) to stuff Sand Veil. Scarf Abomasnow itself is a great check and revenge-killer, to all three suspects at that. Granted, Abomasnow is more of a team commitment than a stand-alone weather user but then so is T-Tar/Hippowdon. :P