[RPG] Enemies AoO generate AoOs? AoOs for non-beginners, understanding Karmic Strike and Robilar’s gambit

Soft Cover

Soft cover is cover, except with the differences that it says. Emphasis mine:

Creatures, even your enemies, can provide you with cover against ranged attacks, giving you a +4 bonus to AC. However, such soft cover provides no bonus on Reflex saves, nor does soft cover allow you to make a Hide check.

That's pretty clear. It says flat out that it provides cover against ranged attacks, with the exceptions listed. AoO's are not listed as an exception, so the Cover rules apply.

AoO's Are Blocked

From the cover rules:

To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).

Here's your example:

C <-> F1 <-> F2

Assuming those are all medium creatures, there is no line between C and F2 that doesn't go through F1's square, so cover applies.

When making a melee attack against an adjacent target, your target has cover if any line from your square to the target’s square goes through a wall (including a low wall). When making a melee attack against a target that isn’t adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.

This says we use the ranged rules for a reach attack, so it has cover.

You can’t execute an attack of opportunity against an opponent with cover relative to you.

Since it has cover, we can't do an AoO.

But! Large Creatures

Now consider this scenario:

---------            1234 = large creature
-12FG----            F & G = medium creatures
-34------
---------

Same thing as before, they're adjacent. Except C is now large. The rules say that a large creature gets to pick one of its squares to determine cover.

To determine whether your target has cover from your ranged attack, choose a corner of your square.

It can use #4 (as it can use any of its squares), and from that square it has the ability to hit G without going through F's square. As a result, G no longer has cover and the large creature can make an AoO.

That's also mentioned in the cover rules:

Any creature with a space larger than 5 feet (1 square) determines cover against melee attacks slightly differently than smaller creatures do. Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee attacks. Similarly, when making a melee attack against such a creature, you can pick any of the squares it occupies to determine if it has cover against you.

On the upside, if G has reach, it can choose #4 as it's square as well, and it can attack the large creature without F being cover.

Sorry, one movement only provokes one AoO from a given defender.

See the d20pfsrd:

If you have the Combat Reflexes feat, you can add your Dexterity modifier to the number of attacks of opportunity you can make in a round. This feat does not let you make more than one attack for a given opportunity, but if the same opponent provokes two attacks of opportunity from you, you could make two separate attacks of opportunity (since each one represents a different opportunity). Moving out of more than one square threatened by the same opponent in the same round doesn't count as more than one opportunity for that opponent. All these attacks are at your full normal attack bonus.

So if you shriek "SERPENTINE!!!" and run circles around your opponent, they still only get one "leaving a threatened square" AoO on you.

The opportunity from Hold the Line is different, but only works once upon them entering your threatened area. So you would get only one from Hold the Line and one for them leaving a threatened square.

Note that the leaving a threatened square is per round - not even just per turn - with the Mythic rules in use in our WotR campaign we've been finding a lot of cases where you get to act multiple times in a round, even charge multiple times, etc. If by hook or crook someone charged you, ran off, charged you again, and ran off again over the course of a round (on their turn or across multiple turns), you would still only get 1 AoO for them leaving your threat but could get multiple from Hold the Line, if they actually leave and enter your threatened area with a charge twice. Up to the limit of the number of AoOs you have.