No valid target?

Can you play a card that doesn't have a valid target (in order to get it into your discard)?

For example, Tachyon wanting to get rid of Sucker Punch when going against Spite (because it's unlikely there will ever be a target with 2HP or less in play).

Yes you could still play Sucker Punch even if there isn't a valid target. There might be a chance of the environment would have a target that gets lowered down to 2 by Tachyon's turn unless she is in hte first hero slot.