Harder bosses (or maybe just Deadbeard) could still be a challenge for me, but you're right, that game was pretty much all about the summons.  Other spells were hardly worth using; it was just a matter of recharging and unleashing over and over again.  I hadn't really considered that as a weakness until you just said it, but it's totally true...

L1 + Square spam in GOW games. It's sometimes the only way to get through some levels on hard difficulties.

Edit: Cyclone of Chaos. 

Wizard/Mage classes.

Its always the wizards that are OP

The smash ball in super smash bros. Now that can be a game changer

The XM8 on Metal Gear Online. It's the most OP weapon in the game.

Selphie's limit break, The End - It can kill an Omega Weapon.

Knights of The Round in FF7
Dragon Crest in FF9 (killing those Grand Dragons not only just to grind, but to power up this attack)

Yojimbo in FFX. Just pay him a certain amount and he'll kill off those Dark Aeons/Secret Bosses.

Just goes to show how many don't know old school OP these days if the BFG 9000 didn't immediately come to mind.

teemo is the imba satan of lol... stupid little piece of shit... so fuckn broken