The End from MGS3.

You can either kill him with a sniper, a while before the fight, when he appears in a cutscene,or during the fight if you save the game and change the date on your system to a few days later (he supposedly dies from old age).

A Spec Ops mission in COD MW2. A friend and I had tried hundreds of times to 3 star it in Co-op but we were never able to do it, getting extremely close on numerous occassions.

Eventually we found a glitch that allowed us to easily take out the juggernauts:
https://www.youtube.com/watch?v=CgRCrhXE7DM

In Age of Empires 2, I randomly typed "Black Death" into the chat box during a random game. It turns out that this was the ultimate cheat code, instantly killing all enemy units. I then proceeded to use this cheat to beat all the scenarios in the game.

Hahaha

The End.

There isn't a video on it, but using the Dragoon class in Bravely Default for the final boss is cheap. Because you can literally attack him by using Jump and he will always attack when you when you are in the air. So yeah, it's pretty cheap.

I was a kid when I first played Final Fantasy VIII and only came to grips with a lot of its mechanics at around the third disk. Once I did...however...ehehehe...

I got Odin by mistake and spent a LOT of hours on Island Closest to Hell, entering and running away from fights, waiting for him to come around and one shot the lvl 100 monsters. Got 3 chars to lvl 100 easy that way: Squall, Rinoa and Zell.
By the time I got to Bahamut and Ultima Weapon, I already had a little trick down.
Triple on one char -> Aura on all chars -> Spam Rinoa's Limit Break until she would cast Invincible Moon. Apparently Aura remains active indefinitely if you're invincible -> Spam Squall and Zell's Limit Breaks (pausing after each combo at Zell for time to identify them, which would give me enough time to land a LOT of combos), with Rinoa hitting with her Wishing Star.
I'm not even ashamed of myself. It made Ultima Weapon feel like a starting area monster. Made Ultimecia feel like a joke of a boss. Also, I avoided learning most of Rinoa's Limit Break attacks, just so I could have a better chance for Invincible Moon and Wishing Star.

Sephiroth in FF7 - In the first fight with Sephiroth, put Fury on all you main 3 (Barrett, Cid and Cloud being best), let them reach their limits, don't use it. Beat Sephiroth phase one to come up against One-Winged Sephiroth. At the start of the fight, cast the limits. Fight over.

FF8 - Meltdown + Aura.

Final Fantasy VI - Equip Gem Box and Economizer, then cast Quick to enter ludicrous mode.

AD&D Shadow Over Mystara - Large Burning Oils against anybody on the ground, especially the Frost Salamander.

I only made it through Demon's soul by abusing every mob's AI. Especially Skelettons.
Also when my brother was loosing a fight in Disgaea 3, he wanted to reset so I took the pad. I still had 1 hero which didn't have enough HP to survive 1 hit from the last mob alive. Ended up chaging his weapon for the only weapon that had more range than the mob, one of the weakest gun in the game. It took me 40 minutes to win and would have taken half that time if he resetted.