I for one have played a ton of Team Fortress 2 back in the day, my favourite class being the spy. Given this background i tried to apply some of my knowledge in Splatoon with varying degrees of success. In this guide i will outline how certain spy tactics can benefit your game (and will allow you to be a great troll).
While shooting people or running them over with a paint roller is a great deal of fun, your objectives usually are different. You have to keep in mind that depending on the state of the game every second can count.
If you are playing a Turf War and you camp a certain location while the enemy team is painting the rest of the map, you will lose. If you are playing Splat Zones and the enemy team is holding the zones while you camp, you will also lose. However, conversely, if your team is having the advantage and you camp a key position that the enemy needs to cross to advance, then you are at a critical advantage: You are not in Zugzwang, thus you can conceal yourself more easily, as you do not need to advance. The enemy on the other hand may become less observant on their way to the front, allowing you to pick them off.
This means that cloak & dagger actions are situational and not something you can use all the time. Likewise you should not optimise your loadout for this in particular, as you may need to be able to hold your own in straight up fights.
Ono ability you will definitely want is Ninja Squid, which allows you to move less visibly. Note that you still leave trails, you just don't make any splashes any more, which are far more obvious. You can eliminate the trails as well by not swimming at full speed.²
Given that you will want to eliminate your enemies quickly your weapon of choice will either be something that deals a ton of damage in one shot, like a roller or blaster, or a weapon that deals a high amounts of damage quickly, like the Aerospray. The .52 Gal is also an excellent choice as it has a fast fire rate and splats unarmored enemies within two shots (~0.151s at point blank range). I would not recommend chargers because they have too much wind-up.¹
Research in the field of abilities suggests that Damage Up is unable to decrease the number of shots needed to splat an enemy using any given weapon, thus it may not be helpful here, as you usually will be the only person dealing damage to your mark.
Swim Speed Up in a primary slot would also be useful to facilitate a faster and easier escape (or to compensate for the speed penalty of Ninja Squid).
Your goal in general is to be not detected, thus Cold Blooded is also a useful ability to have as you will be trackable for a much shorter time. If the enemy uses Echolocator this also has the effect that you can treat your still trackable teammates as decoys.³
Movement & Positioning
The high ground is a great position to be in, from experience people tend to not look up a lot. Of course attacks from above will be less viable against more attentive players. Still, being on the high ground also gives you a better chance to avoid taking fire by backing up. Unlike most other shooters this advantage is limited though, as Splatoon's projectiles have a parabolic trajectory, meaning that they can hit people outside of the line of sight as well.
Walls are a very useful tool as well, since you will also become less of a target when clinging to a wall when the alternative is standing in a ditch. Even with the parabolic trajectory you will usually not be hit when clinging to the wall someone is standing on.
Walls also play a role in terms of what you can expect your enemies to do: In Turf Wars you can assume the ground to get painted; walls on the other hand, not so much. Painting walls is irrelevant to the score and it takes a significant amount of time, hence they are optional targets that get only painted if an enemy is using them, to use them for traversal yourself, or to deny future traversal by the enemy preemptively.
Note that when climbing walls the Ninja Squid ability has no effect (you can also see this, as the ability icon only lights up when swimming on the ground). Letting yourself drift down, however, does not make splashes until the edge at the bottom of the wall.
Some walls provide great attack vectors, e.g. consider this spot on Arowana Mall:
People are likely to walk onto the metal walkway above and shoot down from there, this gives you a chance to come up behind them for an easy kill. Due to the way the wall is facing it unlikely to be hit by enemy ink by default. Walls over which there are metal walkways are great for such ambushes. Urchin Underpass for example features such ledges at the center of the map. They happen to also be prime locations for snipers, which are your easiest targets by far.
Unfortunately you need to spread your own ink everywhere, where you want to use squid form. Thus, you need to take into account where the enemies will paint and where they will expect you to lurk. Here the choice of weapon can be interesting as with splatterguns you can lay down low amounts of ink; with a roller, of course, not so much. The greatest hiding spots in plain sight are Spots that...
- ...contain so little foreign ink, that it would be a waste of time to spray over it.
- ...are out of the common way, e.g. small corners that are inconvenient to paint and not used for fast traveling.
Further, if you have managed to make your enemies paranoid enough they will check behind themselves more often, and depending on which direction they turn you may go unnoticed when coming from the side. As this is a third-person shooter keep in mind that the cone of vision covers more of the sides than it would in a first-person shooter; always be aware of what your enemies will and will not be able to see. And not just see either, swimming makes a lot of noise, and people who pay attention to the sound may hear you coming if you have to swim some distance.
There also is the option of inkless travel & classical camping. If you manage to pull this off without being seen, then it is quite likely to catch enemies off guard, because the lack of enemy ink or seeing one's own ink signals safety.
To give an example, here is an amazing spot on Urchin Underpass:
If you manage to get into that area quickly you can hide behind one of the barrels. As can be seen in the lower image, it it easily possible to get there without having any of your own ink show near the ledge or on the way. This location is especially great because of the no-ink surface, which means that...
- ...enemies will be even less likely to expect anyone there, after all, only morons would stay on no-ink surfaces.
- ...you can easily use this location at any point in the game, because you just need to cross a little bit of enemy ink, and can stay in that position without taking damage.
Baiting, Mind Games & Trickery
While you may not always be able to predict your enemy, you sometimes can nudge them towards making bad decisions. The Sprinkler sub-weapon, for example, makes for a very simple and quite effective bait. A sprinkler placed in the middle of enemy ink is not something that should be completely ignored, they cover a large area if left alone and they do not simply disappear on their own. This can be used to direct enemy attention. You can place a sprinkler to your side and then wait for an enemy to shoot at it, then you can use this fraction of a second to get an advantage by opening fire on them.
This also ties into mind games and mix-ups: A "smart" enemy might think that what i described above is your plan, and cover the area first to find you before focusing on the sprinkler. By doing so his attention is diverted while you might be in his base killing his d00dz. The key is being unpredictable, if your trap is always the same, then any capable enemy will stop falling for it.
Another possible bait is spraying a large amount of ink somewhere and then hiding off to the side in a small puddle. As with the sprinkler, the large ink field will attract more attention and may open the enemy up for an attack.
You can also fake a retreat around a corner and hide up on the wall of said corner. If the enemy chases after you, depending on high the wall is, he might not even see the splashes, and even if he does: You have the advantage in terms of reaction as you can scout around the corner and attack at the perfect time. If the enemy is careful in his approach an actual retreat may be advisable as you will lose your advantage.
Deception can be very situational, sometimes you even need to take your individual enemies' habits into account. Your goal is to find weaknesses and exploit them with the most lethal force you can muster.
- Compensate for lag.
- Chargers can be dangerous if attacked head on. Attack right after your enemy fires (for example to lay down ink to travel in); uncharged they cannot do much damage. Squiffers are dangerous, approaching them head on is very risky.
So far this is all i can think of on the matter. I hope some of it was useful and if you have schemes of your own that you would be willing to share, please do!
¹ Wind-down on the other hand, as you will get from a roller, is not much of an issue, after all, your enemy should be dead and unable to exploit it.
² For more on movement see this excellent guide: Movement Guide 101
³ Suggested by winner133