Whitney is genuinely the strongest leader in G/S and HG/SS. Your mistake was not having a mon with a base speed stat of over 100 and not having a Rock Type. Rock Types resist literally every one of Miltanks moves, as do steel types. Miltank looks pretty fat, but is actually insanely fast. So you either have a very fast, powerful mon: Scyther, Fearow, Fire Starter, Haunter, Kadabra etc, or you have Thunder Wave, or you have a solid tank. Double-Kick is also pretty weak, even though it is "super effective" and probably your best move option for Miltank. Quick comparison:
Double kick does 30 Damage x2, so 60 Damage to a neutral target. Meaning a super effective hit will do 120 Damage.
Quilava's Flame Wheel (learned on lvl 20) is base 60. STAB bonus adds 50%, meaning it goes up to 90. Quilava has higher SpA than Nidorino does Atk, so the damage between the two is almost equal. If "Blaze" is active, the neutral Flame Wheel will actually do more.
I haven't played Pokemon in so long, maybe I'll pick it up again sometimes this year.. I love nerding out over simple things.
j/s, miltank is faster than this boy:
"
Arcanine is known for its high speed. It is said to be capable of running over 6,200 miles in a single day and night. The fire that blazes wildly within this Pokémon's body is its source of power."
"A Pokémon that is described in Chinese legends.
It is said to race at an unbelievable speed."
High speed my ***.
Literally slower than a lactating cow..