If you use scan mode and press RT/R2 while targeting an AC, you will see that AC's stats, including the type of weapons he uses, his AP, mobility and his defense stats. If your scan capability is not that great, (takes too long to scan the enemy, etc.) you will just have to guess based on the type of parts he looks like he has equipped, since it's easy to guess how chassis parts affect defense based on their appearance. (i.e. TE parts are sleek and round, KE parts are blocky, CE parts are kind of wide and angular) Or if you have a teammate with a good scanning head, he/she can probably scan the enemy for you.
Generally defense is just an afterthought to me, because damage racing is virtually non-existent in this game. It takes less than a few seconds to mow down even a heavy or tank AC, given that an AC does absolutely nothing to defend itself, evade or escape. It's also safe to assume that a decent AC will have access to all three weapon types, so it isn't a matter of being lucky enough to have the right counter at the right time. At best, defense will protect you from non-AC type targets and lets you survive just a little longer when facing another AC.
If you want reliable defense, you probably are going to end up with a slight affinity towards one type of weapon while having decent defense in other areas, which I would assume is safer than trying to specialize in a particular defense type or trying to maximize defense all the way just so you can tank an enemy who looks like he only uses dual standard rifles or something.
Having high defense towards a particular area will make most conventional weapons in that category "ineffective" in that they do the least amount of damage compared to other weapon types; but no amount of defense will protect you from kicking, heat piles, kinetic blades, laser blades and Overed Weapons, all of which can shave off at least half of your AP or even kill you outright if you aren't watching out for them.
Something I forgot to add:
Spoiler
There are about 83 Order missions. Order missions extremely short, much like missions in AC4 and ACfA. Combine the story mode of ACV with the Order missions and the game's overall length is the same as any Armored Core game, except that V is way more flexible in letting you do missions on a whim than previous games were.
This was probably already discussed in some detail, but new to Armored Core V are Emergency Missions. These are four-player boss battle missions in a similar vein to Monster Hunter style missions. These will show up randomly and you need intrude points (complete Intrude matches) in order to access them. You can also hire mercenaries if you don't have enough players. Defeating the bosses gets you things like team titles, maybe one or two Overed Weapons, or just a sense of satisfaction. They have a lot of health and can one-shot any and every AC, so it is recommended to come with four people at once.
Here are some examples of the missions:
This was probably already discussed in some detail, but new to Armored Core V are Emergency Missions. These are four-player boss battle missions in a similar vein to Monster Hunter style missions. These will show up randomly and you need intrude points (complete Intrude matches) in order to access them. You can also hire mercenaries if you don't have enough players. Defeating the bosses gets you things like team titles, maybe one or two Overed Weapons, or just a sense of satisfaction. They have a lot of health and can one-shot any and every AC, so it is recommended to come with four people at once.
Here are some examples of the missions:
- Defeat two large warships. The ships have a lot of AP, and a Phalanx CIWS system to stunlock ACs and knock them out of weapons firing range. They can fire rockets that are one-hit kills, as well as vertical missiles that are (highly) explosive and also one-shot kill.
- Defeat a spider-like Arms Fort thing. This is a harder version of the Arms Fort-esque boss you encounter in the story mode. Unlike the story mode version, its has multiple weak spots rather than just one per leg, but only one spot is vulnerable at a time. Also, most of the time the thing would rather spam lasers and missiles rather than use its main weapons, which leaves you with very few windows to destroy their weak spots.
- Defeat Ali-Al Saachez. This is a souped up version of Saachez's Aretha-esque AC from the story mode with multiple large laser cannons and a Primal Armor-like shield. When he uses the lasers the first few times, he flies around in a circle slicing a giant beam of death along the ground; touching it is obviously a one-hit kill. At half of his AP, he starts doing a spinning laser dance which is basically the same thing, except with multiple lasers.