I have a couple of experience with National Dex AG, but not with using FEAR strategy. However from that experience, I can tell a few things about FEAR users:
- Using FEAR is never a good competitive way. FEAR's discussion is totally kept out from AG Discussion threads—they are very gimmicky and requires a lot of reliability on the fact that is your opponent totally unaware of these?
- For example, a decently experience player can guess all of these tricks. He can use counter measures for FEAR—using Dusk Mane Necrozma's Rocky Helmet the time you use Endeavor, or using Sunsteel Strike that ignore FEAR consisting of Sturdy. Hence, I believe that FEAR is something that can be utilized for fun against low ladder. However, using that in real competitive scenario is just ridiculous.
- Like ☆SleepyDreepy☆ said, Rocky Helmet, Leech Seed, Stealth Rocks, Spikes, Toxic, Will-o-Wisp, Sandstorm damage, etc. etc. a thousand of things exist that hamper the proper use of FEAR. However, if you still insist of using FEAR, here are some suggestions (ugh I never want to but for the question...)
Marshadow @ Focus Sash
EVs: 252 Atk / 4 SpD / 252 Spe
- Shadow Sneak
- Close Combat
Marshadow can effectively seen as one of the best FEAR users—it has Ghost-type STAB for those pesky Endeavor immune ghost, and it gets an advantage against some of chip damage users—Poltergeist is unaffected by Rocky Helmet and 2OHKOes Necrozma-Dusk Mane. Shadow Sneak is the quick attack Ghost version. Close Combat hits Ferrothorn, a Iron Barbs and Leech Seed abuser (the most common) in AG.
There are still a lot many—Scrappy Taillow, Rattata, Magnemite, etc. but I don't suggest using those—Marshadow still has some niche as a unresisted STAB hitter, doing both FEAR and acting as a anti-sweep forcer in the tier, which other FEAR Pokemon can't do, as they have slightly to no use outside of this.
I have an recommendation to run with FEAR—Mega-Sableye. Honestly, it is a bit of weird choice. However, it's fully functional and useful. It shuts down status moves, reflects Stealth Rock, Leech Seed, etc. etc because of Magic Bounce. There's a set for it:
Sableye @ Sablenite
EVs: 252 HP / 252 Def / 4 SpD
- Fake Out
- Foul Play
That's all I wanted to say. Hope this helps!