X-Men Origins: Wolverine

For this achievement, you pretty much are going to want to make sure you max out the Samurai mutegen as soon as possible. As soon as you pick it up, equip it and never take it off until you get this achievement. That mutegen provides a multiplier to the experience earned on the reflexes. Try to grab all locations of it as soon as possible.

Samurai Locations:
1) Chapter 1.5: The Escape - During the segment where you are chasing down the scientist, you'll take an elevator down then shortly after catching a glimpse of him running away with a door shutting behind him. Here you will have to fight many machine gunners. After they are dead, you'll then have to pry open the door. If you use your feral senses, you'll see that the scientist ran through more doors on the right. The mutegen is in a room on the left.
2) Chapter 3.3: Betrayal - The last segment ends with a long elevator ride and another flashback segment happens. Immediately as this segment begins, you'll find a rope you'll have to cut to drop a pillar through the floor off to the right. Don't do that yet. Above that in the corner is a platform you can double jump to. On this platform is the mutegen.
3) Chapter 3.9: Aim for the Head - Eventually you'll find yourself entering a control room above the giant sentinel conveyor. In front of you will be a control panel allowing you to control 2 teleporter pads at the same time. The mutegen will be off to the left in this room.

At this point, just worry about finishing the game. WARNING: Do NOT start a new game after beating the final boss. This will reset your progress. Your best bet is to now grind certain checkpoints of the game for specific enemy types. Quick note here, before dropping back to the main menu after completing a segment, make sure you hit a checkpoint first or the progress may not count.

Machete: Chapter 5.1: The Beginning of the End - Trying to max out these guys is a pain. They are extremely plentiful throughout the game, but usually give pathetic experience and, as a result, pathetic progress towards the reflex. At least by chapter 5 the specialty versions of the machete (the spawning master and the flame champion) are worth quite a bit. Make sure to go for the spawners first though because the normal machete guys they spawn are still worth crap.

Machine Gunner: Any Chapter - Really, just worry about maxing out the other reflexes. This one should come while trying to max those out. There are more machine gunners than any other enemy in the game and you run into them almost constantly.

Jungle Mutants: Chapter 5.1: The Beginning of the End - Conveniently, this is the same section as the Machetes allowing you to work on both at the same time. This group basically refers to any enemy during the flashback segments who isn't a machete user or a machine gunner.

Robots: Chapter 3.2: Project Wideawake - First thing to remember is robotic enemies only appear during Chapter 3. The second thing to remember is the highest concentration of the easiest to kill robotic enemies, as well as probably the shortest section in the labs area, is checkpoint 2. As soon as the elevator part starts, drop to the main menu as a checkpoint occurs there.

Specialized Military Units: Chapter 4.3: Please Excuse Our Dust - This group refers to any military unit who isn't a gunner. That means this group contains most of the different types of enemies in the game. This makes this group also fairly easy to complete. You should be nearing the end of this group by the end of the game, but you will likely need just a bit more of a boost. Chapter 4.3 is very good because there is a relatively high density of these enemy types, they are worth a decent amount of experience at this point, and the ledges everywhere make it easy to kill them quickly by throwing them off.