B-air is now my personal favorite aerial. Even more than n-air. For many reasons. Well, the landing lag and start up are turn offs but the range and damage is really what makes b-air. Since the moveset discussion thread won't tackle b-air anytime soon, I'll just preemptively make my post about b-air right now.
B-air
Hitbox active: 18-20 / 21-22
Damage: 12 / 8
FAF: 55
Angle: 361
B-air is pretty much one of Shulk's best moves despite its start up. It's also extremely important to Shulk's spacing due to its tremendous range. B-air has one of the longest range among any move in the game. If well spaced, you can beat out any move in the game using b-air (except invincibility and counters of course). Solely because of its range, b-air has a ton of utility and to top it off, b-air also deals a lot of damage (12% sweetspotted, 8% sourspotted). Be wary of the start up, end lag and landing lag though.
B-air has multiple uses which we'll go over one by one:
1) Spacing
2) Tech chasing/get-up retaliation
3) Ledge trapping
4) Going against campers
For spacing, well this is obvious but anyway. Spacing with b-air is a lot less straight forward than you'd think. The start up makes it feel awkward to use at first. The first thing you need to do to get over it is to learn how to use b-air while FF'ing at the right time such that the hitbox comes out before it disappears via landing lag. But anyway, even if you get the timing of using b-air, you still need to learn how to properly utilize right for spacing. DO NOT SPAM B-AIR OR USE IT OUT OF NO WHERE. If you keep spamming it, foes can easily shield/powershield it and punish the lag quite hard. The best way to utilize b-air is to use it to contest approaches and basically any attack. If you notice your opponent rushing in with a dash attack or an SH aerial, you can perform a retreating b-air to easily outrange or shut down whatever they do. It's also usable for tech chasing because of its sheer range, you can actually catch rolls with b-air if you manage to predict the direction of the roll. Although you'll have to use b-air early due to its start up. The easiest way to catch opponents off their rolls is by maintaining a certain distance from the opponent while they're lying on the ground or while they're teching. You need to keep yourself from a distance such that if ever they roll to the opposite direction, you can catch them off by performing a RAR b-air. If they get up or perform a get-up attack, you can shut that down easily with an SH B-air (short hop a bit forward though to be sure). If they roll at your direction, well.... just b-air them. Using b-air for ledge trapping works the same way as how you deal with tech chasing using b-air. B-air is effective against ledge rolls, ledge get-ups and ledge attacks. It CAN work against ledge hops but that requires timing and precision with the slightly angled hitbox (b-air's hitbox in generally isn't totally straight. Slightly slanted but not that much). RAR b-airs are also a thing against campers. B-air's range is so long, you can thrash through projectiles with it while also hitting opponents in the process if you space it right so in a way, b-air is quite a potent option for rushing through projectiles. It requires a good understanding, precision and timing of its hitbox though.
B-air also lends itself to being good for sealing stocks and gimping opponents. Other than those uses mentioned earlier, b-air's knockback is very impressive. You can use b-air to KO opponents. B-air is also good for edgeguarding. You can use its range to beat out recoveries. You can mix it up with f-air too in case b-air gets too predictable (it's also kinda easy to react to since it's frame 18).
With arts, b-air's utility changes obviously. It has notable changes with buster, jump and speed. In buster, b-air becomes safe on shield. It becomes one of your main tools in your mid-range offense. In addition to that, its increased damage output allows you to destroy more projectiles with on par or less damage output than it. With speed art, basically the increased air mobility and acceleration allows RAR b-airs and retreating b-airs to be much more reliable than usual so it's easier to catch opponents off their tech rolls/get-ups and it's also easier to use RAR b-air against campers because of this. With jump art, you can take advantage of jump's increased recovery capabilities and fall speed when you're off-stage. You can basically try to land b-air off stage without fearing that you won't be able to return back on-stage.
B-air is also the most frequently used aerial with MALLC. MALLC cancels landing lag which solves one of b-air's main issues: the landing lag (21 frames is gross). B-air basically becomes a safe and free-to-use hitbox with MALLC. With MALLC b-air, you can combo off a hitconfirm or even a hit-on-shield with a dash grab or f-tilt or basically any move you can think of. Make sure you space well though because even if you manage to pull off MALLC with b-air, if you execute it up close to the opponent, don't expect yourself to have much options because your frame data is garbage.
B-air is an amazing tool for Shulk. Despite its start up and end lag (plus landing lag), its range is what makes it truly shine. B-air's range alone makes it useful in a lot of situations (ledge trapping, tech chasing, going against campers, spacing, etc.) and the arts further modify the rewards and utility of b-air. Learn to love b-air (like how you love n-air and f-air).