Nope.

EDIT: Giving this a tap towards the top, because only one person has even tried and I really like this one.

The only other "sucking" Boss I can think of is Chaos 6.

Nope. It's not Chaos 6. I guess I can chuck out another hint.

4 - The game was made after Sonic Adventure.

That limits is a bit, and should make it a touch easier.

Is it one of the monsters guarding the pieces of the Preciousstone in Sonic Shuffle?

Nope.

The Egg Golem as Eggman?

Nope.

5 - Not the first time this sort of battle has been used.

Hmm. Black Bull from Shadow?

Nope. Not Black Bull.

Robotnik's space machine o' doom from True Area 53 in Advance 2?

Nope, but that's the closest guess so far.

Okay, it's been a while. Give us another clue =p

6 - Look into the mirror for the help you need.

It's been over a month. I'll post a new one, but I'd REALLY like to hear the answer to the old one.

1. Flaming Phlegm.

2. Tails' end.

3. After a while, he can only give you the finger.

mwaha!
The Final Boss in Sonic and Knuckles!
Giant Robot Eggman!
Dude, whenever I played that boss, I ALWAYS made sure I got rid of the outer fingers so I could see him give me tah finger x3

Elias gets it, but I STILL want to hear the answer to the last one.

Took ya a while, eh? <jk,jk>

Aight, skwit, let's see...my favorite boss...
1) Trapped!
2) 360 evasion!!
3) Bombs away!!!

Meh. I thought I had given up and posted the answer to this. Oh well, here it is.

It's the final boss 'Exception' from Sonic Rush.

1 - Goth baby?
The title of the song used is 'Wrapped In Black'.

2 - This sucks.
Eggman Nega uses an attack that cuts a hole in space which tries to suck you in.

3 - 3 + 2 = Win?
You use Sonic 3 times, Blaze 2, and thus, you've won.

4 - The game was made after Sonic Adventure.
Yep.

5 - Not the first time this sort of battle has been used.
Super Sonic battling an Eggman robot in space. Check.

6 - Look into the mirror for the help you need.
You use both screens of the DS for the battle.

Thanks, Gyser. I'm still stuck on the last Emerald, so I've never seen that boss.

Egg Hornet?

1) Trapped!
You're limited to a circular area to fight this boss, invisible walls won't let you go any farther.
2) 360 evasion!!
When it fires at you the best you can do to evade is run in circles.
3) Bombs away!!!
Well, it fires a lot, though they are missiles..

Nein-
Wrong platform ^_^