Yes shootdown is good vs flying however it has an AoE attack ratio of 5, so you won’t hit a lot of mobs unless you gear up (or use circling). Note your shootdown: falling attribute will NOT make mobs grounded, so ground skills still cannot hit them. Falling attribute will give mobs a debuff. The debuff makes your Cannon Shot deal full damage to them. The only way to make flying mobs into ground mobs is through Hunter’s Snatching.
Cannoneer main damage skill will be Cannon Blast, AoE attack ratio of 7.
You are correct about circling boosting Multishot and cannoneer skills. Falconer Circling is nice when you encounter large groups of enemies and enemies with high AoE def ratio (big mobs). Circling will absolutely boost you and your party during Earth Tower, if you aim for that.
Cannoneer without circling:
with circling:
Multishot + circling:
A2> Sapper2 > Wugu2 > Cannoneer will have good extended AoE, especially vs ground enemies (broom and gu pot). Broom trap and gu pot have infinite AoE so you don’t need circling to boost them.
Important skills: swift step with crit attribute, multishot 10, Stake stockades 10, Broom trap 5, Needle 10, Wugong gu 10, gu pot 5, Cannon Blast (+ other preferred cannon skills).
A3> wugu2 > falcon1 > cannoneer has the benefit of Circling (also a party support skill), and higher burst through multishot + circling, and cannon skills + circling.
Important skills: Swift step with crit attri, Multishot 15, twin arrows 1 (or 5), Needle 10, wugong 10, gu pot5, Circling 5, Call, Cannon blast (+other preferred cannon skills).
A3> Wugu3> cannoneer will have the strongest poisons, and strongest single target damage because of Twin arrows, and maxed poisons for uptime.
Important skills: Swift step with crit attri, multishot 15, twin arrows 1 (or 5), Needle 15, Wugong 15, Gu pot 10, Cannon Blast (+other preferred cannon skills).
High STR will benefit wugushi poisons the most, and high crit benefits cannoneer skills the most.
Circling is a nice support and AoE booster skill, however falcon1 doesn’t offer anything else superb. Falcon1 is taken solely for Circling. It’s up to you to decide whether you find it worth.