My favorite battles are with players who use unconventional move sets and tactics. The biggest benefit is that it helps you balance your teams out better, the other benefit is that it teaches you several unconventional tactics. These unconventional tactics can easily throw a trainer off, while also eliminating the raw power moves' advantage.
Let's look at my Galvantula tactic, which works so well against a Greninja with protean. Greninja with protean is pretty much all I see now, it's predictable, another predictable move is to use ice beam on my Galvantula. The problem is that my Galvantula will stay standing after the first hit, the Greninja is now double ice type and gets hit by my Galvantula's gastroacid. Now the Greninja is stuck in ice, my slower partner for the Galvantula then smacks the frog with a fighting move and knocks it out in one hit.
Yes, it's that simple, my partner for Galvantula is either my Seismitoad, which is never targeted by the Greninja, or something that's weak to ice but can take the damage from one hit. Which depends on if I want my Galvantula later or can afford to sacrifice it. Now look at why this actually works, the player with the Greninja is looking at a bug, thinking ice will KO it with the double STAB, so they don't put any EVs into the defenses, expecting it to sweep in a doubles match with it's superior speed.
The problem with their tactic is that double ice is also extremely weak to fighting moves, so even without STAB the Seismitoad can KO the Greninja with one hit from a drain punch. Thus the only way a Greninja could avoid this is to sacrifice some of the speed and attack EVs to increase the defenses. That is unconventional, however, it is the only thing that stops my tactic in it's tracks, the Greninja survives and still out-speeds my Galvantula, but it still has to switch out to reclaim protean or it will be down in the next round no matter what it tries.
The best effect of unconventional tactics in battles is that they almost always catch the trainer off guard, unless they have encountered that strategy or tactic somewhere they will struggle to figure out a new tactic to counter it. In Battle Spot I was constantly taking out Greninjas with that tactic, and I mean a lot of them. I have other tactics to use against Greninjas in doubles matches as well, because of practicing with other unconventional trainers.
That's the irony of the underused pokemon as well, trainers often only use pokemon with high stats, and this reduces the variety in the battles a lot. It gets redundant after a while, facing the same teams, but when they are facing a pokemon that is not common they panic. They panic because they are unprepared for it, they have nothing to counter it. One example of an underused pokemon that can really catch people off guard is the Sableye with prankster.
Sableye's stats are low, however it's lack of weaknesses make those low stats almost comparable to a Seismitoad. Add prankster and a good set of status moves that trainers won't expect and you will have the advantage. Drop a few status effects, especially one of the major ones like sleep, onto their toughest tanks and you can just chip away their hitpoints in your own time. Or hit it with a bunch of stat changes.
Though stat changes can be reset by switching, you have taken their advantage away from them whether they switch or not. If you know the other pokemon in their team, you can spam attacks those are weak too on the weakened ones. These attacks will still do some damage, more if you weaken it well enough, and when they switch in the next one you will likely hit it with a super effective move.
Changing status values can be devastating if you can predict your opponent's actions well, thus the common tactics and pokemon become a disadvantage no matter how high their stats are. If you are lucky then even without good predictions, you will still be at an advantage with the correct status moves.
This brings us to my favorite tactic, shifting chance to your side. Paralysis and sleep are great luck based statuses to inflict, it gives you time to buff up or switch pokemon without much risk. Confusion is another good luck status to cause, though not always the best because it can be cured by switching. The goal of this strategy is to decrease their chances of being able to use any moves, then you can switch in some heavy hitters or sacrifice those with the status moves and cause some damage to keep the edge regardless of losing one or two pokemon.
The sacrifice play is one often overlooked, it's valuable for many reasons, one is that it avoids switching into a damage move, the other is that the opponent has to use PP to bring them down, valuable PP. If your typing is done right, they will not have enough big damage moves to bring your tanks down before they can claim your victory.
Typing is another factor often missed, with the return of STAB this can really hurt your team. Often I will find a team that the trainer has too many of one type of damaging move, and none of another. My team is balanced in type weaknesses, at most two or three of my pokemon will have a type as a weakness unless I suspect they will not have that type of move available. Also the most common types used are best avoided as weaknesses, which this changes depending on which pokemon is the most popular of the week.
The best rule of thumb, if it's popular, avoid using it. That is the ultimate tactic, don't use a common one because the best trainers will be prepared for it, but do prepare for the most common weaknesses for your team. This means you need more than six pokemon ready for battle, so you can change your team when needed. You also have to practice different strategies all the time, get use to your teams, all of them. Don't waste too much time on one single team, or you will lose the advantage over the overused pokemon.
I hope this article helps some new trainers to offer up more challenges in the near future. I don't mind losing, as long as you don't cheat, and I hope you will offer something new to see. Also, never overlook the underdogs, even an underdog can bring you down.