Get your bio ball (upgraded with stim + concussive shells) up as soon as possible. Also, check what units opponent is concentrating on. You wouldn't want too many marauders against zealots.

If there's only one Collosus, you'll still manage. If you see 2 or more, then back off and build a couple of vikings. Starport with reactor is essential for quick switching between Medivacs and Vikings.

When you're approaching mid-game, get your ghosts (that said, it's better if you get it out as soon as possible to allow energy charging) to accompany your bio ball + vikings.

Last but not least, please remember to upgrade your units whenever possible.