The categories for research are:
Thuggish Splicer (splicers with melee weapons)
Leadhead Splicer (Splicers with guns)
Nitro Splicer (Splicers with Grenades)
Houdini Splicer (Splicers with fire that teleport)
Spider Splicer (Crawls on the ceiling & has claws)
Rosie (Big Daddy with the rivet gun)
Bouncer (Big Daddy with the drill)
Security Bot (the flying guys)
1) Have the freezing or electrobolt plasmid equipped with the camera selected as your weapon. Freeze or electrocute the enemy, then take a picture to minimize the amount of damage.
2) There are tonics you can equip to increase your research score on each of these and they will cut down on the number of photos you have to take a lot.
3) There are two Houdini Splicers who dance in Cohen's apartment - this is a great place to max out your research
4) You get diminishing returns if you photo the same subject several times, but with the Little Sisters and Big Daddies you won't have a choice
5) Once you max research on the security bots and turrets, you get an insta hack on them - which is awesome
6) Once you hack a turret, bot, or camera, you can no longer take pictures for research since friendly units don't count
7) You get a bonus if you shoot the Big Daddy and get the Little Sister in the same frame or vice versa.
8) You get a bonus for multiple subjects in the same shot.
9) At the end of the game, before going to take the final boss down, save your game, go back and try for a picture of everything to see what you haven't maxed your research on.
This achievement will pop as soon as you 100% research all items. You do not have to wait until the end of the game.