For this achievement you must pass Mechat Shooting Stage 3 while taking NO damage. This stage occurs on Disc 3.
First off you will want to upgrade your mechat with all the extra parts, see my solution here:
for locations and what you need to get them.
I'm basing this guide on that you have fully upgraded the mechat.
The Landshark starts off right in front of you, keep firing with your machine gun and take out the mines while hitting the shark with your missiles. After taking about 3/4 of its health it will move to your left side. At this point he will fire off some missiles so keep on the machine gun, he will then move behind you, which he well then like to fire off two missiles: one left and one right, just keeping laying on the machine gun and missiles until his health is gone.
A short cut scene will occur and then another health bar will pop up. At this point lay into him with all of your missiles and keep that machine going as he will start shooting of 4-5 rocket groups and splitting down both sides of the screen, also he will occasionally fire 10 missiles straight at you so KEEP THAT MACHINE GUN FIRING. Taking out the missiles are your top priority so take them out then focus on hitting the shark.
As long as you upgraded your Mechat your machine gun will never overheat so don't ever stop firing it.
Note: Also BE SURE that when you kill the shark that there are no missiles left as once it dies if there are any missiles left in the sky they WILL hit you and you will fail, it's happened to me twice and boy will it make you angry.
This may take a few tries even with an upgraded Mechat, if you don't upgrade your Mechat you will be in for a very stressful time.
The achievement will pop soon after the shark dies.