Good to see a Dark Eldar / Eldar battle report
A lot of ruling mistakes though (some of them might be me not understanding correctly what you wrote):
My list included
HQ: Farseer - Both Runes
HQ: Autarch - Reaper Launcher / Chainsword / Mandiblasters
HQ: Warlocks (3)
You cannot take 3
HQ under 2000 pts. And if you are above 2000 pts and take 3 or 4
HQ, you need at least 4 troops.
His list included
HQ: Archon - Shadow Field / Agonizer / Blast Pistol
...
Elite: Incubi (5)
DT: Venom - Upgrade to second Splinter Cannon / Night Shields
You cannot fit 5 Incubi + 1 Archon in a Venom, whose transport capacity is 5.
It went down due to HP damage from glances but it passed the saves agaisnt the hits that rolled on the table (First Blood).
First blood is earned at the first killed UNIT, not model (wouldn't have changed the final score, the first unit killed being the reavers... should have made 1 unit of 6 bikes!). The Venom is not a separate unit, it is a dedicated transport (part of the unit).
It hit, and wrecked the vehicle since it was open-topped
What's the link between being open-topped and being wrecked more easily? An open-topped vehicle suffers a +1 malus on the Vehicle Damage Table, not on its penetration roll. Thus it explodes more easily, but does not lose
HP more easily (in your case it didn't explode since you didn't mention any damage to the embarked troop).
Farseer guided one of the Avenger squads and targeted the Archon with Mind War
Mind War is a focused witchfire, but it can target a specific model only on a 5- on the psychic test, otherwise the targeted model is randomly selected (just a remark, maybe you rolled a 5- on the psychic test). Moreover, this is subject to debate, but nothing prevents an independant character or a character to make a "look out, sir!" in response to this power. This is sad, really, but no errata was published for now, and the precision shot rule (which does not concern focused witchfire but is similar) explicitly states that "look out, sir!" rolls are allowed..
sucks when the roll of six needed to hit in overwatch also makes the weapon AP2
The roll of 6 to hit on overwatch does not make shuriken weapons AP2, because shuriken weapons are made AP2 on a 6 to WOUND (bladestorm rule, similar to rending).
Total score at the end of the game was
Dark Eldar: 4 (3 from kill points, 1 from Slay the Warlord)
Eldar: 14 (9 from kill points, 1 from the fleeing squad leaving the game, 1 from Slay the Warlord, and 3 from the Objective per the mission rules).
In an Emperor's Will mission, no points are earned for killing units, so the final score would be:
Dark Eldar: 1 (1 from Slay the Warlord)
Eldar: 5 (1 from first blood, 1 from Slay the Warlord, and 3 from the Objective per the mission rules).
Plus, you annihilated the Dark Eldar army, so this is actually an auto-win (no need to calculate victory points). Plus you would have had 1 or 2 turns left to get the other objective and the "line breaker" secondary objective for an additional 4 points