All Levels 99.. The Konami Way. 

there was an OCD guy who ran around afghanistan & africa just to farm S-rank soldiers..
he also hoped that he can gain S+ to S++ soldiers from PF points..

and today, he realized that it was waaaayyyyy much more easier to farm S+ to S++ soldiers
via the Skulls event..     

not to mention easy farming of GMP, Haoma & Digitalis too.. 
being an OCD gamer ain't easy.. 

---

btw, jellybean-fish, the Skulls event gets easier after a few rounds..
i opted to repeat the Medical Platform to farm the plants..
equipped myself with a Ze'ev Stun Lvl 5 & Serval Lvl 5,
spawned onto the point nearest to the AA Gun - fulton 'em,
then stunned 3 guards with the Ze'ev & fultoned them too..

wait for the Skulls to appear, and spam them with the Serval..
just be careful with the Skull's offense (be ready to avoid those exploding rock-stuffs),
and u'd be able to get all tasks done easy.

E to C, do you mean the PF Rank?

there are 2 different rankings in the iDroid Mother Base menu :

[attachmentid=6335643]

Espg. Rank (Espionage Rank) - this is the rank which would change, based on your FOB infiltration exploits, normally showing numbers & the insignia.

PF Rank - this is the ranking for the virtual PF battles, the ones that are set weekly and you win/lose Offensive/Defensive virtual battles, based on the strength level of your staffs/motherbase/resources/vehicles/usage of Liquid Carbon Missiles or Anti-Ballistic Missiles.



from the PF battles, you earn points too, and eventually able to redeem soldiers/resources with those points (although the redemption bonuses aren't that lucrative as FOB Events).

[attachmentid=6335702]