Can I use Defiant Dance if my enemy only has Ruinrunner on the field?

Yes, you can play Defiant Dance if your opponent only has a Ruin Runner on the field, provided you have at least two of your own units to target.

According to RiftJudge FAQ #2002:

• Defiant Dance requires two distinct unit targets to be played.
• Ruin Runner cannot be chosen as a target due to its ability ("I can't be chosen by enemy spells and abilities").
• If you cannot select an opposing unit due to Ruin Runner, you cannot target it. However, if you have two of your own units on the board, you can target both of them instead to satisfy the spell's requirements.

If you do not have two other units of your own to target, you cannot play Defiant Dance because you would be unable to select the required two distinct targets.