5-10 CM Executor S2 Main DPS | Arknights
BEWARE: The following is just me ranting. :)
5-10 is, in my opinion, the best map for Executor. Besides the defense crushers, most enemies have very low defense, making Executor's S2 very viable, since it's great at straight up deleting trash mobs. Executor's range is also complimented well in this map, which is a big plus.
Still, I recommend bringing a lot of SP batteries + Elysium whenever trying to make Executor's S2 work (Or you could just use S1, but it's honestly not very fun to use in my opinion.)
As a general rule of thumb for whenever to use Executor, I always make sure to bring along Liskarm and Ptilopsis, since their synergy with each other is insane. It can bring down Executor's S2 cost from 70 down to 30-40, which makes the skill easier to use. Elysium is a big plus, since he shreds defense making it easier for Executor to handle high defense enemies.
Also, I don't have much experience with wide range snipers, but in my opinion Andreana is the best one. Even at E1 max level, her stats are insane, with S2 reaching over 2k damage per hit. There is the penalty of the skill which doesn't allow her to hit enemies below 50% HP, but I don't think it's that bad of a penalty. If anything, it just means she'll be hitting several enemies instead of wasting all of her shots on one, applying a slow and also weakening several enemies for other units to finish off. Plus, she's got to be AT LEAST an A-tier waifu, and she's git drip. What isn't there to like about her?