NOTE: All these strats assume no chance for Thunderbolt. If he can Thunderbolt, you can't ever predict his moves, so a perfect setup isn't achievable. So do general hope to live and land a GI strats.

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Pikachu has 186 HP
Will Heal at 45 or Below. Thus, 141 Recoil is too much. So don't do over 140 with the first two hits. 140 is OK.

Aim to do ~113-140 instead, and then have it kill Snorlax from full.

A half HP Gyarados & 2 Snorlax kills generally gets there 
(Snorlax matters more)
2 Snorlax + Kenya + Quilava always gets there.
L31 Kenya + Max Recoil Gyarados (237+) + Max Recoil HP Snorlax (234+) gets there.

Snorlax has 220 - 235 HP. This is 73 - 78 Recoil.
Gyarados has 216 - 238 HP. This is 72 - 79 Recoil
At Half, this mean 108 - 119 HP. This is 36 - 39 HP.
L31 Kenya has 87 HP, therefore 29 recoil (28 if L30, 27 if L29)
Quilava has 55 - 51 HP. This is 18 - 20 Recoil.
Sentret has 13 - 16 HP. This is 4 - 5 Recoil.


Gyarados usually comes in at ~80% after Blue.

So, math whether Gyarados + Snorlax will put to heal. E.g. Gyarados at 180 HP is 60, and Snorlax at 230 HP is 76. 60 + 76 = 136, does not put to heal.

Also math whether half Gyarados + 2 Snorlax will kill to be sure.

Almost never want to heal Gyarados after Blue, but if somehow below half you may want to Super Potion.

If Snorlax has bad Defense, AND Current Gyarados HP + Snorlax will put to heal AND Gyarados + Snorlax + Kenya will kill, then heal Gyarados to full.

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If 100% VT (Great Defense):

If god HP on both + L31+ Kenya:
Lead Snorlax, switch to Gyarados
Send out Snorlax, switch to Kenya
Send out Snorlax, Max Revive Gyarados. Die, get Blastoise.

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Gyarados + Snorlax won't put to heal HP:

Lead Snorlax, switching to Gyarados.
Send back out Snorlax, Max Revive Gyarados
Send out slave, Max Revive Snorlax
Send out Snorlax, die, get Blastoise.

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Gyarados + Snorlax will put to heal HP:

Lead Snorlax, switch to Kenya.
Send out Snorlax, switch to Quilava.
Send out Snorlax, revive a slave.
Send out slave, Max Revive Snorlax.
Send out Snorlax, die, get Blastoise.

OR

Lead Snorlax, switch to Gyarados
Send out slave, Max Revive Gyarados.
Send out Gyarados, switch to slave.
Send out Snorlax, Giga impact to kill.

(slightly riskier but faster as have to hit a GI)

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If in IT range, (Medium-Bad Defense)

Gyarados + Snorlax won't put to heal:

Lead Gyarados, die.
-
IF VT: Go Snorlax, Max Revive Gyarados.
Send out slave, Max Revive Snorlax
Send out Snorlax, die.

-
IF TB: Go Snorlax, Revive Gyarados
Send out slave, Max Revive Snorlax
Send out Snorlax, switch to Gyarados
Send out Snorlax, use Crunch (If either def gonna die, Rev)
Max Revive Gyarados as Blastoise kills.

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Gyarados + Snorlax will put to heal:
Lead Gyarados, die
-
IF VT: Go Snorlax, Max Revive Gyarados
Send out Gyarados: X Speed, WF.

OR If Gyarados + Snorlax + Kenya will kill.
Go Snorlax, switch to Kenya
Send out Snorlax, Max Revive Gyarados.
Send out slave, Max Revive Snorlax.
Send out Snorlax, die, get Blastoise.

-
IF TB: Go Snorlax, Revive Gyarados
Send out slave, Max Revive Snorlax
Send out Snorlax, switch to Gyarados
Send out Snorlax, Crunch. (If either def gonna die, Rev)
Max Revive Gyarados as Blastoise kills.

OR If god HP + L30+ Kenya:
Go Snorlax, switch to Kenya
Send out Snorlax, (Max) Revive Gyarados.
Send out slave, Max Revive Snorlax.
Send out Snorlax, die, get Blastoise.

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Snorlax thresholds:

81 Defense or less: Can always get Iron Tail at +0

82: Max roll is 233
83: Max roll is 230
84: Max roll is 227
85: Max roll is 224
86: Max roll is 222

88+: Max roll is 217 or lower, so never chooses.

-

116 Sp Def or better: Max roll is 219 or lower, so never chooses.

115: Max roll is 220
114: Max roll is 222
113: Max roll is 225
112: Max roll is 226
111: Max roll is 228
110: Max roll is 231
109: Max roll is 232
108: Max roll is 234

107-: Max roll is 237 or higher, so can always get TB.