let's do our best; come on!; we can do this!
Volitional form of 頑張る used as an encouraging interjection. Implies a shared effort or mutual support, like 'let's give it our all' or 'we've got this'.
Let's all do our best!
Just a little more, let's push through.
Volitional form of the verb 頑張る (to persevere, to do one's best). The interjection usage derives from the inclusive 'let's do our best' meaning.