Card says:

"If you have a die showing (do all that apply) :
• Ranged - Deal 1 damage to a character.
• Focus - Turn one of your dice to any side.
• Discard - Discard a random card from a player's hand."

Can these be done in any order? Or is the effect simultaneous? Or is the effect resolved in the order shown (top to bottom)?

E.g. If I play this card and only have a Focus showing, can I use 'Strike Back' to turn a die to a side showing ranged, then do 1 damage (or turn a die to a discard and resolve the Discard effect of 'Strike Back')

No.

You resolve the effects on the card in order. So you could turn a die to a discard with a focus and then do the discard related effect but not the ranged as that effect has already happened.