One of the most effective and cheap ways to defeat bosses on 9.0 difficulty is to combine some of the strongest powers in the game. These being Energy Charge, Weak Point Reticle, and Playing Dead. With these powers, you can stay invisible and Invincible (as long as you do not move around), auto target the weak point on the boss and with Energy Charge, your offensive power increases immensely, getting stronger and staying effective until Pit gets hit. With invincibility from Playing Dead, and Energy Charge and Weak Point Reticle active, you can just hold down L to auto attack the enemy For Massive Damage, and kill the boss quickly and easily. This works better if you have a weapon with great rapid fire, like Beam Claws. Most people use this strategy to beat the chapters on 9.0 with ease. You can even use this strategy to easily destroy people on Multiplayer. The only time this doesn't work is if a chapter has circumstances set up where you can't use powers.
Clubs in Free-For-Alls, as they have immense power and reach, providing easy KOs.
Freezing. While under this status ailment, you are at the complete and utter mercy of everything gunning for you. Unless it's a Light vs. Dark match wherein a teammate rushes to your aid in time, you are dead, end of story.
Petrify to a lesser extent, since that at least gives you a defense boost while Taken for Granite; Freezing does not. And considering the power of whatever you're being hit with, even that defense boost might not save you.
Trade-Off far from implies Equivalent Exchange. Sure, you're reduced to minimal health, but you're not going to care, not when you get flat invincibility along with the ridiculous offense and movement boosts for twenty seconds.
The invincibility combo. (Trade-Off, Brief Invincibility, and Playing Dead.) Any of these are overpowered on their own, due to providing invincibility for no real penalty, but when you put them together, the combo abuser will be impossible to touch for most of the match, while Trade-Off provides them free points. And God help you if the invincibility abuser becomes the angel when your own team's angel isn't in yet.
The Taurus Arm is even worse. While it doesn't quite have the reach of the Magnus Club, it has more melee power than nearly any other weapon in the game, and rather high mobility to go with it.
Beam Claws. It may not have the best charge shot, but it still makes you decently fast and its rapid fire is just one Large Beam that goes on for a very long time. It's like you have the Mega Laser power equipped, except you can use it infinitely.
Laser Staff. If you can get a mod on it to have good defense and range, you can do absurd damage to people on multiplayer, nearly one shotting them and to easily destroy enemies on 9.0.
Slipshot power. The ability to shoot through walls is far more useful than it looks to be.
For more fun, combine Slipshot and a weapon with good Homing (for example Eyetrack Orbitars) and a homing boost mod, and a mod and stats that increases attack power. Now go hide behind a wall and shoot through it, suddenly dealing massive damage without warning.
For that matter, Homing boost in general, namely on what are usually Painfully Slow Projectiles. Example: the Black Club has a very slow charge shot, but it lasts longer than any other in the game and already has decent homing itself. With a Homing boost modifier, those gigantic black blocks are suddenly doing U-turns after hitting/missing you so they can come back to hit you again.
Aries Armor. It provides a ridiculous defense power boost and immunity to stunning and launching, literally turning a player into The Juggernaut.
Having a good arm (that means not using Autoreticle) turns the Flintlock Staff into a potential Game Breaker. Its charge shot goes from point A to point B almost instantly just like a real firearm, and staves gain more power with more distance between the shooter and the target, which it has the most of out of all the weapons in the game. With six stars in its Ranged stat and a good Charge Shot modifier, it's easy to constantly OHKO people from across the map and will even take a good chunk out of an Aries Armor user. The charge shot also has innate Shot Cancellation.
As far as Solo mode goes, a Twinbellows Cannon with a freezing mod is lethal in air battles. This is a weapon that, by default, can apply burning to opponents. And since the power of flight turns any continuous fire bar clubs into machine-gun fire (sorry), this basically means that when you hear that freezing echo, you can change targets, because you are undoubtedly going to have burned them by then. If you can maneuver your angel through Bullet Hell, even a 9.0 Intensity air battle is easy. Just don't get smart for the ground battles. Youwillregret it.
In Multiplayer, the combination of Lightweight, Transparency, and a weapon with good melee damage is highly effective and very difficult to counter. Transparency ensures that the player will have no difficulty getting to within melee range and will almost certainly be at high health at the start of the duel, while Lightweight's speed boost and stamina loss negation allows the player to cover ridiculous amounts of ground before Transparency runs out, which means that the defense decrease is nearly completely negligible, with the exception of area of effect attacks. With a decent melee-oriented weapon, such as clubs, arms, or claws, it's entirely possible to pull off a Total Party Kill within SECONDS of the match beginning. Worst of all, this combination becomes Hero Killer incarnate for Light vs. Dark mode, since this tactic allows the player to sneak up on the enemy's angel and wipe out at least half of their health before they even know that he's there unless the angel goes Properly Paranoid with defensive buffs. Even then, however, players using this tactic get to refill both powers at the start of every life, while the angel cannot, meaning that this technique will not stop, everuntil the angel is dead. And even if the player using this tactic dies at the wrong time and becomes an angel himself, likely robbing him of his good melee weapon, he can STILL use both tactics to evade enemy fire long enough for his teammates to finish the job.
The Power Attack modifier is a Glass Cannon example. Having Power Attack +4 (the highest possible) on a weapon usually means you won't get much in the way of stats and other modifiers, but the trade off is that it doubles the strength of Powers like Mega Laser, Explosive Flame, Meteor Shower, Land Mine, and so on. What were once panic buttons become lethal surprise attacks, and the modifier on a weapon with decent damage and/or an innate Speed boost makes for a good one-two punch KO for whatever survives the Power spam. Granted, you have to die to get your Powers refreshed after using them all up, but done right you should be gaining more than you're losing.