This can only happen if Zigzagoon is generated with 11 Attack, and the wild Ralts has a Nature that lowers Defense, an IV of 3 or less in HP, and 5 or less in Defense. With all this, Zigzagoon can do a maximum of 10 damage with its first Tackle, and after Growl is used on it, 7 with its second. This makes for a total of 17 damage, just enough to knock out Ralts.

1/8 (HP IV)
3/16 (Defense IV)
1/256 (2 Max Rolls)
4/25 (-Defense Nature)
91/400 (11 Attack Zig)
361/400 (Hit 2 Tackles)

98,553/32,768,000,000 or ~1/332,491