Diamond Hollow II

Diamond Hollow II is an online 2D Flash Platformer/Shooter created by Arkeus of Pegas Games.

You begin by finding yourself stuck at the bottom of a large cavern with no clue as to how you got there and you don't remember who you are. Befriending the first thing you see, an sentient gun named Silo, you begin your long and arduous ascent from the caverns to the surface. You mission is clear: shoot, kill, explore the caverns, collect gems, use gems to upgrade your skills and gun powers, conquer foes, take out fierce bosses, and acquire many achievements all in a desperate effort to escape from the nefarious clutches of the evil Professor who is trying to take you both out.

This game can be played at here or here.

Tropes used in Diamond Hollow II include:

A Boy and His X: Or in this case, an amnesiac boy and his intelligent gun.
Awesome Yet Practical: The spread gun. Why bother with focusing a small enemy with a tiny bullet?
Bittersweet Ending: At the end after defeating the Professor in combat, before SAGA becomes reprogrammed by the Professor, he activates an emergency elevator which takes Silo all the way up from the Professor's laboratory to the surface. Afterwards, SAGA lets out one last Big No before succumbing to the Professor - leaving Silo to be the only known survivor of the facility. This was only one chapter after SAGA expressed much hope for the both of them to live a more non-violent and peaceful life...
Blob Monster: The basic enemies and the first boss in the game.
Boss Rush: The titular Boss Rush Mode, which allows you to sequentially fight all seven bosses of the game in order. The Heroic difficulty setting ups the ante dramatically by making all of the bosses significantly stronger and more advanced.
Convection, Schmonvection: You can be quite close to lava and still not get killed or damaged in the least. However, if you touch it without the Molten Suit...
Crate Expectations: Every chapter has wooden crates scattered across the level. In the later chapters, metal boxes (which can only be destroyed with missiles) get added into the mix. Both either hamper your way, and/or provide gems when destroyed.
Double Jump: The Sky Boots allow you to do this. They are later upgraded with the Moon Boots, which then allow you to triple jump.
Endless Game: Escape Mode, Runner Mode, and Tadpole Mode go on indefinitely and keep getting harder and harder as you keep on progressing until death.
Experience Points: Diamonds serve this purpose.
Expy: The boy looks a lot like Cloud Strife. And there's a certain similarity between them...
Final Boss: Subverted initially: the first Professor you fight is a decoy. Rather obvious, as he is entirely coloured in a weird black hue, he doesn't say anything, and the fight occurs in chapter 6 of 7.
Flunky Boss: All of them. In many cases, the flunkies are actually more dangerous than the boss themselves, in quantity if not in quality.
Friendly Fireproof: None of the enemies can be harmed by spikes or lava.
The Goomba: Slimes.
Heart Container: 17 in total. They are all spread out and hidden among the 7 chapters, and allow you to increase your max health to 20.
Heroic Sacrifice: SAGA, after the Final Boss fight, who sacrifices themselves for Silo to escape.
Invincible Minor Minion: All of the golden plants are indestructible. Thankfully, they only spawn during boss battles.
King Mook: A lot of the bosses.
Lava Is Boiling Kool-Aid: With the Molten Suit, you can survive under lava for a short period of time while taking non instant-kill levels of damage. Though, it's much more effective in Story Mode than in Escape Mode.
Lava Pit: A few of them can be found throughout the game - especially around the latter chapters.
Mad Scientist: The Professor. He created all the monsters you face, the underground tunnels, and SAGA, dumped Silo at the bottom of the cave for some reason.
Nintendo Hard: The higher elevations of Escape Mode, the longer distances in both Runner Mode and Tadpole Mode, and the Boss Rush Mode's Heroic difficulty setting.
One-Hit Kill: All of the Lava Pits will instantly kill you if you don't have possession of the Molten Suit.
Palette Swap: All varieties of slime are palette-swaps of each other. Some of their abilities vary though.
Retraux: This game has graphics and audio quality and style circa of the late 16-bit or post-SNES era.
Rise to the Challenge: The main hazard of Escape Mode, where lava rises from the bottom of the stage and you must keep climbing upwards (while collecting gems, and taking care of the Mooks) to escape it. The higher you get, the tougher the Mooks become and the faster the lava rises.
Shout-Out: There are a ton in the achievement names, mainly to Portal 2.
Spread Shot: What the Spreader Gun's specialty is. It starts out by firing a spread of 3 bullets forward. It can be upgraded to fire a spread of 5 bullets and have the center spreader bullet do double damage.
Turns Red: Bosses have at least 3 different phases, each one harder than the previous one.
Underground Level: The entirety of the game takes place several stories underground (save for the ending).