To allow its power to scale, let Spike-o-Pult one-shot Bloons. During the early stages of Spike-o-Pult, pop weakened bloons. As its ATK increases, it becomes easier to immediately one-shot with a Spike-o-Pult without needing other Monkeys to help out.
Initially, Spike-o-Pult starts off weak, but if given enough rounds and ammo to scale its ATK, it will be difficult to stop without using removals such as Shrink or Return to Sender, and it is useless against OTK decks.
Giving Spike-o-Pult extra ammo (Temporary Ammo, Try this!, etc.) allows it to pop more Bloons and scale faster.
The Eternal's health scales slightly faster than Spike-o-Pult's ATK without extra ammo.
Avoid overrelying on a scaled Spike-o-Pult for damage. Most competent decks with a focused wincon will have many ways to neutralize or bypass scaling Monkeys like Spike-o-Pult.