The Chansey must be bred in generation 3, where it can learn Mimic through move tutor. Transfer it to gen 5 or gen 6. Keep it at level 5.

In a triple battle, have Chansey use Mimic to get Flame Charge. Use stat changing moves to get Chansey’s attack to -6 and the opponent, a level 100 Carracosta’s Defense to +6. Use Will-o-Wisp on Chansey, then skill swap it to have Defeatist and Carracosta to have Thick Fat. Deal enough damage to Chansey for Defeatist to activate. Use Rain Dance and Water Sport. Switch both of Carracosta’s allies into Pokémon with the ability Friend Guard, have the opponents set up Reflect. Let Chansey use Flame Charge on Carracosta, if it resulted in a minimum damage roll, it would deal about 1/301 damage. But of course, it would always get rounded up to 1 damage.

If you recall from Fact 35, the formula for damage is:

[(2L+10)/250 x P x A/D + 2] x M

L is the attacker’s level

P is the move’s base power

A is the attacker’s (special) attack stat, including stat changes

D is the defender’s (special) defense, including stat changes

M includes other factors, such as STAB, type effectiveness, weather, critical hits, etc.

Using Constrict on Stakataka is not the answer here, because base power and defense both comes before the “+2”, making their nerfs near irrelevant. To get minimum damage, we need to get the M value as low as possible, and if you recall from fact 34, fire type moves has the most nerfs. Of course, all of those nerfs are part of the M value. The changing power of Eruption is part of the P value, so it wouldn’t work.