ἂν μὴ τῷ ὅλῳ ὑπολάβωσι τοῦ παντὸς κρείττους εἶναι:
I understand it as ‘overall,’ ‘completely’ but the translation renders it weirdly as ‘that the whole of their force.’
“Overall,” “completely” would make τοῦ παντός redundant. The translation is right. Instrumental or comitative dative.
The commentary says that τοῦ παντός refers to the other army.