My team is
Bellsprout Lv. 20
Vine Whip Growth PoisonPowder Sleep Powder
Hoothoot Lv. 19
Tackle Growl Foresight Peck
Quilava Lv. 21
Quick Attack Leer Smokescreen Ember
Slowpoke Lv. 18
Curse Tackle Growl Water Gun
P.S. Miltank's Stomp is to strong
Both Croconaw and Geodude might be better than both Machop and Gastly. Croconaw can learn fury cutter by TM, and fury cutter becomes very powerful if you use it against her Clefairy a few times. Geodude has higher physical defense and resists all of Miltank's attacks, and it learns magnitude at level 16. Water and ground also end up being more useful types than fighting, and the fact that poison and ghost attacks are all physical means Gastly is very bad at using STAB attacks, so Croconaw and Geodude will be better than Machop and Gastly later in the game. source source
There are several ways you could go about this: