My team is
Bellsprout Lv. 20
Hoothoot Lv. 19
Quilava Lv. 21
Slowpoke Lv. 18
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.
There are several ways you could go about this: