My main badge hunter is a Sonic/Invulnerability Sentinel, and I've not found any content that I cannot contribute meaningfully towards including "Master of" trials. As Sonic you are actually better in a team because your resist debuffs make everyone else hit harder. You can pretty much keep 9.6% -res active at all times, on top of the standard 5% -res for Sentinel attacks, and that's without offensive opportunity adding another 20%. Its like you have Achilles' Heel and Annihilation procs rolled right into your kit.
Here's my take on the abilities in the primary:
1) Shriek - Needed for offensive opportunity. Slotted with just Superior Sentinel's Ward and with Hasten it does 89.31 s&e DPA on about a 1s cooldown (!), plus multiple res debuffs.
2) Scream - I prefer offensive opportunity to defensive, and the 5 pulse DoT is a bit slow for my Hasten build. Its good while levelling.
3) Howl - Cheats on cone rules (10 targets max) and the best non-T9 power to spread the -res across packs of mobs. Slightly less damage than Shriek vs one target but more vs 2 or more. A good mule for Superior Opportunity Strikes.
4) Shout - Long activation time makes this very poor DPA. Its OK until you get Screech but then respec out of it. Cool kettle / train whistle noise though!
5) Amplify - It takes Gaussian's Synchronized Fire-Control, so why not? The cooldown lines up pretty well with Dreadful Wail so pop them together in sequence.
6) Shockwave - My opinion of the entire Sonic set massively changed when I put Overwhelming Force knockback to knockdown here. Mag ~5 knockdown that I've got on about a 2s recharge time, so I can keep entire packs floored (which stops Fake Nemesis from being able to use their invulnerability before I defeat them). Active damage mitigation and team utility. Doesn't deal -res but that's why we pair it with Howl.
7) Sirens Song - I don't take this, but I did look at it for a proc monster power if I wanted one: 20s base recharge, and it takes damage procs from Annihilation, Bombardment, Javelin Volley, and Positron's Blast. I'd also Hami-O the range so that it matches up with the other two cones.
8 ) Screech - Even though I've slotted it defensively with Superior Frozen Blast for hitting softcaps, this still does 242.55 damage (161.70 DPA) on a 3.2s cooldown, plus a magnitude 3 stun, and 12 seconds of the -res debuff. The stun lasts over 9.5 seconds so these can be stacked quickly to overcome bosses.
9) Dreadful Wail - Sentinels are pretty somewhat defined, for better or worse, by our ability to spam our T9 nukes. This PBAoE slotted with Superior Avalanche does 338.40 damage (mostly energy) and I've got it down to a 25 second recharge. That is on top of a mag 3 stun and 20 seconds of the -res debuff, and typically with increased to-hit, damage, range, and Gaussian's proc from Amplify (that is on the same recharge).
All damage numbers are before buffs (excluding Hasten). I put a lot of my build budget into hitting defence caps and permahaste, so I am not running attack procs except my Interface incarnates. I do have the Psionic Mastery epic pool but I mostly just use it for Link Minds and against enemies that pop Moment of Glory, Mind Probe usually deals less damage than Screech. Against packs I mostly use Dreadful Wail, Shockwave (KB>KD), and Howl, and against single targets its Dreadful Wail, Screech, and Shriek.
In summary, Sonic isn't a "moot" set; it brings unique utility on leagues, and when solo the game is easy enough that you can win with any powerset.
