Step 1. Identify the challenge the PCs were supposed to overcome in this fight.
Step 2. Determine if they did, or to what degree they did.
Step 3. Find CR of encounter and determine the experience rewarded by overcoming the challenge to the degree that they did.
Step 4. Alter this reward based on any circumstance modifiers you see fit to (did they have an advantage, a real disadvantage?)
Step 5. Split up, and dole out experience.
I think, looking at this, you might realize that it's the Assassins who should be recieving experience from this encounter rather than the PCs.
If the challenge for the party was simply to keep the one character alive and they failed, it's hard to award any experience... if the challenge was simply for the character in danger to survive taking damage and making fort saves, ew. While I respect the need for proactive NPCs, as a player who has too been killed by assassins I can say that no amount of experience dished out would be needed- because all I was taught was that I could be thwonked by DM run NPCs at any time. Not a lesson I needed.