Note that the entire DLC must be played through without missing a single enemy. It has been confirmed that restarts work. Innocents can be harmed when getting this achievement, since I did my two chievs separately on separate restarts of this level (I would just select restart after the score was tallied, and it let me get the other achievement once I had fulfilled the requirement). This is quite hard; not only do you have to memorize the levels, but you have to make up for the shoddy shotty mechanics. Here are a few tips:
The spread widens as it leaves the barrel, meaning that it is much harder to spray far enemies accurately than close ones. Don't shoot if they're close to an innocent; chances are the innocent will go away before the baddie does.
Make sure to move your hand straight up and straight down. Anything that the reticule touches as you are firing will get hit, and while sometimes useful to utilize a 'diagonal shot', generally avoid it.
Once you have the levels memorized, make sure to plant your cross-hairs early on each enemy, before they pop. This will save you valuable time, and some go away quite fast.
Lastly, you can shoot over the heads of innocents once they start lowering. Their timing window will have closed, and you are safe from killing them.
Spots I got tripped up in the first stage:
Row of 4X3 boxes in the first stage: Three rows of enemies spawn, then a row of innocents, and the a vertical row of enemies on the left that goes away very fast, then a vertical row of innocents, and then a vertical row of enemies toward the right. Almost always, when I aimed at the bottom guy and swiped straight up, I got everyone. However, I have been doing a second shot right after the first just in case, and it saved me on my perfect playthrough of the first level.
Last boat at the end: There is a boat right at the end where two planes will be flying at each other, and two innocents will pop on the left, two baddies on the right. EVEN WHEN SHOOTING THE PLANES, make sure that you only aim at an area over the right side of the boat. I nearly broke something failing at this point in the level.
Point of note in the second level:
Approaching the wall:
This is hard because there are some buggy things here that stalled me for hours on the same parts. When you reach the wall, you will pause, then start strafing left. Soon after the first airplane flies overhead two shotgun girls will spawn next to each other. You have about 1.5 seconds to shoot them before two innocents spawn in front of them. If you don't take them out prior to that, chances are you aren't going to get them. You have to be EXTREMELY careful as you strafe along the wall, as I think it's the easiest place to blow both achievements. There are hard to see innocents, and when the plane flies by at the end, shoot it quickly, because innocents will spawn near your blast zone. Don't forget to check the upper wall for the far away shotgun girls.
As you start to back away from the wall, a somewhat glitchy set of bads spawn, 3 on either side. It will be your first spawn after the 5 innocents appear in a line slightly to the left. You need to wait a second for the right side group to fully pop up (the guy on top comes last) before blowing them away and then QUICKLY shoot the left side group. The reason this has to be done quickly is that it's possible to register a miss here EVEN IF YOU HIT ALL THE TARGETS by shooting the left side late. I personally saw my target explode and then register a miss at least three times. Assuming you have done enough full runthroughs of this level to memorize, you are now nearly in the clear. Just be careful!
Hot air balloons: Memorize the spawn sequence; some enemies don't stay up long enough if you are inefficient with your aiming, so instead of waiting for innocents to despawn, make the most of your time by knowing where it will be safe to shoot on the screen, i.e. shooting the two top left shotgun girls while the innocent is despawning down on the right hand side, then clearing the right side out. After this, you reach a point right before the boss (as you go up a small ramp and enter her bedroom) where you will see a couple of groups of bads protected by innocents in front. The innocents will despawn in time to shoot the bads, but not by much. The only way I could fully ensure registering all the kills was to start shooting bads as the innocents lowered.
Boss woman: Not hard. Just be super careful in the auto fire sequence to go around the edges of the screen to get to the bad girl (holding the mini-gun), and during the last section when she pops up with her innocent dupes, be on your toes. You can see the tip of the barrel of the gun as the innocents are despawning in front of her, one row before she is revealed. All in all, not hard.
All auto fire sequences:
Take your time. I can't emphasize this enough. The reticule likes to jump when moving even at moderate speeds from side to side, and this could cost you here. Even if the sequences are not yet memorized, you have plenty of time before each one ends, so make wise use of that time and go carefully.