stand fast, hold firm, withstand, hold out, resist, stand pat, stand fire, strong-arm, tank, square up, more...

yield, give in, give way, submit

unyielding, resolute, adamant, obdurate, inflexible, intractable, more...

put one's foot down,   stand strong,   refuse to fold,   put your foot down,   hold your ground,   don't back down,   stick to one's guns,   dig in one's heels,   to not budge,   foot down