One thing I really like in Armored Core For Answer is the wide variety of enemy types that the player must fight across the game's many missions. Each mission offers a distinct combination of different enemies, from the puny MTs to the colossal Arm Forts, and as a result the game presents a variety of challenges the player needs to overcome. This is particularly important considering the game's focus on NEXT customizations, since each type of enemy requires different strategies and weaponry. While a skilled sniper NEXT armed with nothing but long-range rifles will work surprisingly well against an enemy NEXT, its slow rate of fire and limited ammo makes it a somewhat poor choice against large numbers of MTs or Normals, and most Arm Forts could ignore its attacks completely. The difference in enemy types encourages the player to experiment with NEXT customization, and makes each stage an entirely different experience than the last.
Anyways, here are some thoughts on each major enemy type:
Normals: These are the most basic enemy grunts in the game. They move around at a slow pace, but have boosters and can fly if need be, and some can even be quite fast. They can take a few hits, but not many. Some are powerful long-range snipers, while others carry melee weapons. Just about anything works on them, but they can pose a threat regardless. These guys are just tough enough to make high attack-power weapons like blades and plasma cannons meaningful, but common enough to make ammo matter. They are great opponents, since they balance being tough enough to be significant with being weak enough to come in large numbers so well.
MTs: These are the weakest enemy type, coming in somewhat large numbers but always at very low power. If they move, they hardly do so, and most can't fly and will die if you send them falling into the water or something. They are supposed to serve as cheap cannon fodder that can be destroyed in vast numbers, but most of the time they don't show up in big numbers at all, with the exception of only two stages or so (Escorting the Red Berets and the attack on Cabracan). Also, their total lack of mobility makes them seem too much like fixed guns, rather than an actual vehicle. I can't say that they are really used to their fullest in the game.
Ships and Traditional Aircraft: These show up in several missions, but they feel almost out of place alongside the less realistic Normals, NEXTs, and Arm Forts. I do like a lot of the aircraft, since they either fill the role the MTs should have, or act like larger, aerial Normals. On the other hand, the ships seem too much like immobile floating gun turrets, are mostly too small, and suffer horribly in comparison to the Arm Forts. Still, all of these kind of craft serve as a good excuse to use the heavy weaponry of the game, without being too much of a threat to the player, so they can be a lot of fun.
Small Arm Forts: These mostly include the Arm Forts like the Land Crab or the Eclipse, the smaller kinds that show up several times across the game. Other than the fact that they are often too easily destroyed (some seem to go down as quickly as an enemy Normal), they have an impressive presence on the battlefield due to their large size, impressive weaponry, and large numbers of support Normals, so they can have a strong impact on a mission. They are always a lot of fun, and one of my few complaints about them is that there are simply not enough missions in the game where you have to fight off a large number of these things. Also, these things really are on a much smaller scale than the larger Arm Forts, and fill a very different role in the game, so I wonder if they really should share the same name...
Large Arm Forts: These things are true monsters, and are always a stage in of themselves. They tower above the landscape and possess truly frightening destructive power, which makes any battle against them a memorable experience. As far as I am aware, For Answer is the first game in the Armored Core series to have such enemies, and they make a fantastic addition. Any problems with them is limited to specifics about each one, though those issues can be significant. For example, one of the strongest and most notable Arm Forts, the Answerer, requires an excessively specific kind of NEXT configuration in order to fight, doesn't broadcast its weaknesses well, and the fight against it ends too suddenly and anti-climactically. Still, the concept is sound, and it was certainly a good idea to put them in such a prominent position in the game's story and achievement selection.
NEXTs: This is easily the most diverse category of foes, and a particularly important one since almost every named character in the game is a NEXT pilot. NEXTs are unbelievably fast, possess Primal Armor capability, and have incredibly accurate and powerful attacks of every imaginable type, which means that they are both uniquely powerful opponents and can avoid or negate many kinds of attack that would be effective against lesser enemies. To top it all off, even an average NEXT has more resistance to damage than an Arm Fort. Ultimately, every enemy NEXT is built with the same system that the player's NEXT is built with, so enemy NEXTs fight with all the same power that the player uses to wipe out Arm Forts and destroy dozens of Normals. These guys can be very frustrating opponents, and missions in which you have to fight NEXTs can be terribly repetitive, but the game would not be anywhere near as fun without them.