Tupi-Guarani is a broad language group, not really a cohesive nation.
The Jesuits set up Reductions where the Guaraní were educated and shielded from a lot of the worst aspects of colonialism. They armed the Guaraní with European weapons and tactics to repel the invasions of the Brazilian Bandeirantes. Guaraní even had their own artillery so they’d actually have a solid unit roster unlike most native factions.
Sepé was overall commander of the indigenous army, which numbered nearly 1,700 men and fielded eight indigenous artillery pieces made of bound tacuaruzu cane that could be fired only a few times.