To unlock this achievement, you must get gold on 50 of the 54 available challenge levels. You unlock new challenges by completing a certain number of other challenge levels (each locked level tells how many more you need). To unlock new stages, you do not need to get gold medals, just complete them before the bronze timer runs out.
The great news about this achievement, half the work is done for you. Simply check the leaderboards for any given level and watch the replays of the top players. This will show you most of the time saving and efficient strategies. Even if you mess up a few times, you should still have room to make gold. Be warned though, even with all the strategies laid out for you, the execution can be quite daunting.
Here are a few notes concerning things you may see in replays that may cause some confusion, so hopefully this will clear things up:
Pause buffering: In the original Mega Man, there is a popular exploit that lets you pause the game (LB) at certain points to allow weapons to deal extra hits on boss enemies. Although I thought I was ahead of the curve knowing this trick, turns out it is all but required on many Mega Man challenges in order to get gold.
Mega Man 2 boss skip: On a couple of the boss stages, namely Heat Man and Bubble Man, there is an exploit that allows you to pass the stage if you hit the boss while touching the entrance door. It is a little tricky, but if you see it done in the replays, pay close attention and you should be able to duplicate it.
Autofire: If you see a replay in which a player seems to be firing at an unusual rate of speed, it is because they are using autofire button (initially Y button). It is very useful in some places, but the charged buster shot is more useful in others. If I was in a stretch where I needed to use autofire frequently, I would change the control layout so it was assigned to "X" button.
Death Glitch: There is a glitch where if you die during a challenge, and part of your dispersing energy (mega man exploding upon death) touches the exit portal, it will carry you onto the next area. If you fall in spikes, you will start the next area with the same life bar you had before. If you die due to energy depletion, you start next area with empty life bar. Sometimes, an empty lifebar grants you invincibility, most of the time though, you die after the next hit.
Now, as far as gold times go, there doesn't seem to be any indicator in-game that tells you each goal. Below is a list of all the challenges, and my time for gold. This is not a definitive cut-off time for gold medals, but may give you a reference if you think you are close:
MM1 Remix 1: 1:54:78
MM2 Remix 1: 2:25:86
Yellow Devil: 1:22:26 (without pause buffering)
Mecha Dragon: 19:15
Craig's Challenge: 2:43:68
MM1 Remix 2: 3:34:68
MM2 Remix 2: 3:51:05
MM1 Robot Rush: 1:37:06
Wily Machine #1: 38:03
MM1 Megamix: 3:58:43
MM2 Robot Rush: 2:04:65
MM2 Megamix: 4:36:50
Wily Machine #2: 29:80
MM1-2 Mashup: 5:25:71
MM3 Remix 1: 2:06:15
MM4 Remix 1: 2:02:00
Yellow Devil MK-II: 44:88
Square Machine: 56:96
MM3 Remix 2: 2:56:61
MM4 Remix 2: 4:38:33
MM3 Robot Rush: 2:37:85
Wily Machine #3: 31:53
Cossack Catcher: 32:60
MM3 Megamix: 3:51:15
MM1-3 Mashup: 5:18:06
MM4 Robot Rush: 3:49:21
MM4 Megamix: 5:11:70
MM1-4 Mashup: 5:58:20
Wily Machine #4: 46:98
MM5 Remix 1: 2:45:00
MM6 Remix 1: 2:23:91
Big Pets: 26:40
Mechazaurus: 24:70
MM5 Remix 2: 5:52:91
MM6 Remix 2: 4:39:28
MM5 Robot Rush: 3:26:05
MM5 Megamix: 5:06:03
Wily Machine #5: 1:33:58
MM1-5 Mashup: 6:41:55
MM6 Robot Rush: 3:39:21
MM6 Megamix: 4:09:95
Wily Machine #6: 51:20
MM1-6 Mashup: 6:16
All Appearing Blocks: 2:16:58
Dr. Wily Forever: 7:00:10
All Robot Rush: 20:00
MM1 Robot Rush (No items): 3:08:96
MM2 Robot Rush (No items): 5:39:46
MM3 Robot Rush (No items): 7:30:00
MM4 Robot Rush (No items): 5:56:51
MM5 Robot Rush (No items): 6:04:16
MM6 Robot Rush (No items): 6:04:53
All Appearing Blocks (No items): 3:02:80
All Robot Rush (No items): 45:00:00
Hope this helps. If you have any questions concerning any of the challenges, please post in the comments and I will do my best to answer. Thanks.