Almost right away.
1) Sneak attack (as an archer)
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Get someone next to his target (melee them) and he can do a ranged sneak
attack. Sneak damage is underwhelming at level 2, but just gets better
and better (2d6 at level 3, 3d6 at level 5, and rising). Also, get him to
high ground whenever possible to give him a +2 on his range attacks. The
guy is literally "death from above."
Between he and the melee character, they can deal a lot of damage very
quickly. Especially if it's a raging barbarian doing reckless attacks.
Once he gets the invisibility spell (level 2), he can confer advantage on
this without a spotter. But he won't get level 2 spells until he is 7th
level, so that's a bit of a wait. Since invisibility only lasts rounds
instead of 1 hour as in core 5e, getting advantage in combat is about all
it's good for. You'll probably want to save it for Misty Step (see below)
anyway.
He can do cunning action hide too if he's not in a vision cone. More
advantage, more sneak.
Have other characters blind things and that also confers advantage
(Shadowheart is good for this). More ways to do sneak damage.
2) Defending Astarion
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So now we have to keep him from getting cold cocked by someone.
Get him shield (level 1 spell) as soon as you hit level 3 and he is very
hard to hit. Shield is a reaction. It can be taken any time, even out of
his round. As soon as someone scores a hit he can instantly raise his AC
by 5 points for the entire round. Make sure you have that reaction set to
confirm, because sometimes you may just want to take the damage.
Keep him away from melees because that confers disadvantage on his ranged
attacks. If there's no way to avoid that, melee sneak attack is also
pretty good for murder. I think you can get him Misty Step (level 2
spell) for those situations too, which is an up to 30' teleportation to
any line of sight location.
And absolutely, at level 4, get him the "Lucky" feat, which gives him the
ability (3 times per long rest) to make attackers reroll and confer
advantage on picking and disarming. Save it for crits scored on him. It's
a big deal. Do NOT put his DEX to 18.
3) Spells
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Cantrips - Blade Ward and Dancing Lights. Ranged cantrips are useless,
since he's going to sneak bow everything. Blade Ward gives him resistance
(1/2 damage) against common melee damage. I believe Dancing Lights
confers advantage against anything inside it, which you can use if you
can't melee something with another character. Then it's the flavor
cantrip. Mage Hand? Sure. Friends to talk your way out of a theft? Why
not.
Level 1 - Shield, Shield, and Shield. That and Mage Armor. Keep him from
getting hit.
Level 2 - Scorching Ray? By now you should have a handle on this build. I
haven't gotten this far with him so I have little to no advice.
...
So he's best used as a sniper. Really good at it actually. He is far and
away the best companion to do high DC lockpick and trap disarm, but the
sniper ability makes him very deadly.
--
Zag
No one ever said on their deathbed, 'Gee, I wish I had
spent more time alone with my computer.' ~Dan(i) Bunten