Edit: 5/12/16: Added Alternative Early Game, Ability info, and Item suggestions.
Edit: 5/15/16: Tweaked build for overall better late game and dueling viability. Improved crit and lifesteal for a slightly lower damage per hit, should increase end game power and survivability. Also changed Agoran Scepter build path as per suggestion. Thanks for the advice @Timo. And added link to deckbuilder, as per request.
Edit: 5/26/2016: WOW. Okay so huge changes this patch! Crit was heavily nerfed, and the prevalent opinion right now is that it isn't worth building for optimal dps output. So I have updated my build to reflect these changes and give you guys a quality build that has worked for me post-patch. I will keep testing new builds and keep the guide updated to reflect changes in the meta.
Link to Agora.gg Deckbuilder
My name is Faolon, you may know me from my Sevarog guide, but probably not. I'm making this Grim guide in order to show you guys what has worked for me.
GRIM.exe is a character that does great damage throughout the mid and late game, along with having outstanding defensive capabilities. His autoattacks ramp up in scaling as you level them, although I am not sure of the exact numbers at this time. Generally you build him how you would build Murdock. Damage>Crit>CritDamage>Att Speed>Lifesteal, at least this is my formula.
Overcharge : Grim's passive. This is the ability that gives you fantastic burst throughout the entire game. It provides bonus damage on your autoattack every 8 seconds. It appears to have some sort of scaling, although I am not sure what it is. Late game your passive hits for around an additional 400 damage. Making your autoattack the hardest hitting in the game.
Pulse Cannon : Your standard autoattack. Improves scaling with each level, make sure to level it.
Displacement Blast : This ability is an AOE displacement. It can be fantastic for getting yourself out of a bad situation of hitting someone in travel mode. However, this ability can be harder to aim because it is lobbed. One thing to note is that the direction that it knocks people in seems to be more or less random. I've tested it every which way and the direction it knocks the enemy in seems to be very inconsistent, so I try not to rely on this too much. Late game it does decent damage, but it's nothing special.
Deflector Shield : This ability is what really makes Grim feel like a MOBA champion. Ability blocking shields have been a staple in most MOBAS, (Sivir anyone?) and it is especially good on Grim. Because of your low movement speed and large hitbox, it can often save you from deadly CC or ultimates. Keep in mind that the duration is not that long and the cooldown is, so try to use it only when you need to. It is worth noting that there appears to be a bug where sometimes the shields visual effect will remain and the ability itself will not activate.
Suppression Mode: Another amazing ability that works well with Grim's kit over all. It slows the enemies movement speed but costs mana per attack with it toggled on. It will eat your mana quickly, so use it sparingly. It is amazing for kiting and chasing. One possible recommendation is to change out Curse of the Leech for Sirens Song. It leads to a slightly lower damage, but higher situational survivability.
G.T.F.O : The ability says it all. Late game this baby will hit for over a thousand damage and can never miss. The only downside is that it can be body blocked. It's worth noting that you don't need line of sight on an enemy to use it, you can target it through terrain. Another thing is that the range is fairly decent, roughly the distance from the inner turret to the outer turret.
Abilities upgrade priority
I've been upgrading the displacement orb, pulse cannon, suppression mode, and G.T.F.O. to max. The only thing I don't put more than one point in is Deflector shield
Things to note about abilities
Grims Displacement Blast seems to work somewhat erratically. You can shoot it between yourself and an enemy and they'll fly behind you. As a general rule of thumb, it seems that the orb often launches them the opposite way that you would think. I can't believe that this is intended and I hope that this gets fixed.
There seems to be a misconception that his Deflector Shield has multiple uses. This is simply not the case. The utility of the ability is outstanding and being able to block an ability can be absolutely pivotal, however that is all it can do. This ability does not block auto attacks, minion damage, or tower shots.
Suppression Mode is an ability that allows you to kite and chase other characters. This ability really is lackluster until it's maxed out though, as the slow percentage is quite low. Personally I may experiment with maxing this ability first, as maxing Displacement Blast doesn't seem to increase its power much at all. All in all it doesn't seem like a bad option.
Grims ultimate ability G.T.F.O has a bit more usage than people might think. While it is effective to wait until an opponent gets low and then ult them, it can also be effective to initiate a fight with the ability. Given the nature of the ability, it makes for an excellent initiator. It is a fast projectile that can never miss, meaning that it is excellent for snaring people in travel mode and catching them out. Also using the ability at the beginning of a fight when you have a clear shot means that you will guarantee your damage isn't blocked by another person.
Stay back and farm, as your damage output isn't that great. You definitely can win in one on one situations, but it often isn't worth the risk.
This is the stage of the game that you are most vulnerable, if you are caught out 2 v 1 then you will most certainly die. Grim has a very low movement speed and a very large hitbox, this makes you an easy target for ganks. Early game you will do very little damage compared to most other hunters, as you have no reliable damaging abilities until you have your ult.
Alternative Early Game
I've only had the chance to try it once, however, if you can build a team around it, Grim jungle could be pretty good in theory. His kit would allow him to gank safely and effectively, with both a displacement ability and a constant slow, his kit makes him a very effective ganker, and his ult ensure that he can finish off his opponents or catch them out if they're in travel mode.
Stay with your team and take objectives. You are excellent in a 1 v 1 and as long as you stay in the backline in teamfights you're untouchable.
Stay with your team and win the game. You should be critting for around 1K damage not counting the damage from your passive. In addition to this you will be shooting faster than Murdock. You should be melting everyone as long as you stay near the backline.
With the changes to crit it seems that it isn't worth building. Or at least not as worth it for consistent and optimal dps.
Currently I'm building a Magus ward first and following it up with several whirling wands. I round off the build with a shadow scroll and curse of the leech. The focus of marksmen is much more mid-game oriented as far as taking control goes, however they will still destroy late game with proper positioning. The current state of the game is much more balanced than it was previously, and rangers have to actually be careful and watch where they are in order to stay alive and deal good damage. They are still the masters of consistent and sustained dps in the late game, however they can no longer assault people with reckless abandon. I feel this patch over-all helped game balance, but at the same time it ruined it by making crit seemingly pointless for most builds.
GRIM.exe is an amazing marksman with great carry potential. Much like Sparrow he requires great positioning to be used to his full potential, but if you can manage to play him well then you will surely dominate your games. If you have questions, comments, or concerns, please leave them in the comments and I'll try to update regularly. Hopefully this guide will help someone or even inspire a better build. Thanks for stopping by, and happy hunting.