Arcade Achievements Guide

Load up the first level (The UAC) on one of the easier difficulties, like I'm Too Young to Die. Collect both bonus lives floating in the air in the later parts of the level, and grab all six relics, which will pop out of each of the level's six floating barrels when you blow 'em up. Upon completion, this will net you 8 bonus lives/items. Rinse and repeat two more times, and half-way through the third try you'll earn the achievement.

Get 600,000 points playing The UAC on I'm Too Young to Die. You'll need to kill everything you can as fast as you can, grab all the extra lives (Doom Buddies) and shoot all the floating barrels, and make sure you grab all the relics as soon as they pop out of them, because the point bonuses you will get for collecting each relic will diminish with time. The multiplier will double every time you get a kill or at least deal an enemy damage while the multiplier gauge is full, and while picking up items can also increase the gauge, only kills and combat can double it. The gauge and multiplier will also decrease with time, so try to chain your kills as much as possible. You will still collect any health, armor, ammo, and extra life pickups you pass through, even if you're full on them, because they're also there to help maintain your multiplier gauge level. You will get extra points for Glory Kills, Multi-Kills, Super-Kills, Barrel Kills, Deaths from Above, Headshots, Mid-Air Kills, Rocket/Explosive Shotgun kills, and Triple Hits, not to mention Gold Relics and Extra Lives.

Full disclosure, the main reason I made this guide was because Twenty Sevens posted a great guide to Filling the Trophy Case on TrueAchievements, and I wanted it readily available on my overlay so I could nab the achievement myself. So I post it here, in its entirety:

Arcade Mode
Level: Kadingir Sanctum
Difficulty: I'm too young to die

-Medals-

Multi Kill:
Kill a cluster of lesser demons using a grenade or other explosives.
(Imps, Unwilling, Possessed Scientists, etc.)

Super Kill:
Once you reach the area with the Baron fighting the horde of Unwilling, throw a grenade in that general area. The resulting explosion should kill enough Unwilling to net you a Super Kill. If you don't get a Super Kill here, kill all the enemies so the door unlocks and you can progress. Down the hill pass the door, there is another group of Unwilling. Killing them with a grenade should earn you a Super Kill.

Barrel Kill:
Kill a demon by shooting a barrel or a mine.

Glory Kill:
Perform a Glory Kill while and enemy is stunned (flashing orange).

Death From Above:
While an enemy is stunned (flashing orange), double jump above them and execute a glory kill.

Head Shot:
I recommend using the Heavy Assault Rifle's Tactical Scope mod for pinpoint head shot accuracy. But if you're not a fan of the rifle and prefer to get up close and personal, you can also do this with the shotgun.

Stunned Kill:
Bring up the weapon wheel and equip the Plasma Rifle. Then press up the the D-Pad to attach the Stun Bomb mod. Simply hit a demon with the Stun Bomb (Left Trigger) and kill it while it is stunned.

Mid-Air Kill:
I find it easiest to kill Imps while they are jumping up to a ledge, or across a gap. I recommend using Siege Mode on the Gauss Cannon so there is no missing.

Direct Hit Kill:
Kill a demon with a direct hit from the Rocket Launcher, or Explosive shot from the Combat Shotgun.

Triple Hit:
While using the Combat Shotgun, hit up on the D-Pad to attach the Charge Burst. Press and hold Left Trigger to initiate alt-fire mode, and then fire the gun like normal. Landing all three shots on lesser demons should kill them and net you the medal.

Gold Relic:
Shooting the mines causes them to explode and launch a relic. The relic stays gold for a few seconds, so collect it before it changes color to get this medal.

Silver Relic:
Same as the Gold Relic medal, but wait for the relic to turn silver before collecting it.

Bronze Relic:
After the relic turns silver wait a little longer for it to turn bronze and then collect it.

Extra Life:
Collecting Doomguy bobble heads grants you extra lives. They now have green auras and are found floating in new locations.