The killer wail is very good, in my opinion. It's biggest strength over other specials is that it only takes 160 points to charge, because of this, it helps get early lead if you can get a few kills with it or just keep opponents away from a certain area, giving your team a head start sometimes. Using it to get rid of long ranged gun users at high vantage points is also extremely useful, since it forces them to jump off, losing their advantageous spot and possibly forcing them into a bad position, making them much easier to kill. Another great use for it is just locking down the opponent's spawn if your team just needs a bit of time for the final push for the lead, doing this has saved games for me before. In general, you want to just force your opponent away from certain spots on the map.
The levels that I'd say it's best for are as follows: Urchin Underpass, Arowana Mall, Bluefin Depot, Camp Triggerfish, Port Mackerel, and Walleye Warehouse. It's still very useful on the other levels, but there just aren't as many great spots to use it in, from my experience.
Turf War: Use it to keep your opponent's from covering ground as much as possible.
Splat Zones: Use it to keep your opponent's away from the zones as much as possible, anytime you opponent is in a better position your team, use this to get them away from it.
Tower Control: Use it to get your opponents off the tower or keep them away from getting to the tower by locking down their their spawn point.
Rainmaker: Use it as a wall to keep the rainmaker from moving forward, or making a safe path for your team's rainmaker.
