Gnar has a seemingly mismatched kit, with abilities scaling with AD, AP, and health, meaning more often then not people will devote themselves to a single build path, building as much AD, AP, or health/resistances as they can, but this leaves him easily countered, and thus, a rather weak pick in most people's eyes.

Gnar is a great champion, but his kit is and should be focused on utility, either by out maneuvering your opponents in mini form, or locking them down in mega form. your damage should be coming from gnar's passive shred and auto attacks, and his abilities should be used to keep him alive either by keeping enemies at range, or locking up enemies up close.

as Gnar's passive would otherwise make him very unbalanced, his base stats and scaling are poor compared to other top laners, even boosting the worst health scaling of any champion. because of this he is very weak early game, but scales very well into the late game with his passive, so runes are focused on giving him as much defense as possible, and bringing him into higher levels as quickly as possible.

Attack speed Marks will allow you to proc hyper as much as possible, melting opponents and allowing you to escape from counter attacks with the move speed boost.

Armor Seals / MR Glyths will keep you alive in the early game, as your defenses are extremely poor otherwise.

Exp Quintessences will allow you to push past early levels faster, when you're at your weakest, and get you into more favorable levels, hopefully above your lane opponent.

all of Gnar's main build items are in some way focused on doing on hit and passive damage as much as possible. when it comes to Gnar, the more attacks you can do, the better.

Blade of the Ruined King - this is arguably one of the best AD items in the game, and this is especially true with Gnar. the smooth build path, diverse boosts, and powerful passive give Gnar a large power spike. definitely a must have.

Zephyr - while this is a great item, very few people use it as it lacks the potent passives of other items. it does however have the extremely rare tenacity passive, which is amazing in it's own right, and can only otherwise be obtained by mercury treads. this is an item that gives incredible potential, and can even replace purchasing boots, but when combined with boots, makes you one of the most mobile champions in the game.

Wit's end OR Black Cleaver. your choice here should either be a niche pick or counter pick to the enemy team. Wit's end gives your passive hyper an even more deadly bite by increasing the rate you can proc it and stealing the enemies magic resist, but it doesn't help any of your other abilities or attacks, as they all do physical damage. Wit's end is best taken with or against a team with an AP focus. Black cleaver on the other hand greatly boosts the destructive power of all of your abilities, and shreds armor, giving you and your team a greater damage potential over time, but also limit's the power of your passive, and gives you a less consistent damage output. Black Cleaver is best taken with or against an AD focus.

Boots - while ultimately boots are entirely a personal preference, they can also be completely replaced by Zephyr, leaving extra room for another powerful item, or can be bought purely to fill a given roll. i'd recommend either boots of mobility or swiftness to compliment Zephyr and to allow you to roam and kite more effectively. i wouldn't recommend berserker's greaves or mercury treads, as you're already building everything they provide with other items.

final late game purchases should focus on a balance of damage and sustain, either via health (Frozen Mallet), or Magic Resist (Maw of Malmortius).

Gnar's skills are heavily utility based, so using them effectively will do wonders for your chasing and kiting potential.

Gnar's Q is a secondary damage skill and by far gives him the most range, but it has much greater uses then just last hitting minions and poking. it's great utility comes from procing his passive hyper. if used correctly, in can be used to either create distance from an enemy, slowing them and giving you a large move speed boost, or it can be used as a gap closer in near the same way. using your boomerang properly is the key to dictating the flow of the battle. just remember that the boomerang will always line up with where your running when it returns, so you can control the path it comes back on. in his mega form, it does substantially increased damage in an aoe, but doesn't return, making it great for a chasing tool or a point blank wave clear.

Gnar's W is his primary damage tool, and gives his kit ton's of utility through the massive move speed boost it gives. while different from the rest of his kit as the only form of magic damage he has, the % health shred is enormous, and being able to proc it effectively will keep you out of trouble and allow you to go in on enemies while doing crazy damage no matter how much health they build. in his mega form, it forms the opposite roll, giving him the ability to stun all enemies in a large area in front of him for a fairly long time, and doing heavy physical damage.

Gnar's E is his utility skill, and serves as his most potent engage and disengage, rivaling that of lee sin's ward jump's and Q. he can hop on anyone to double the distance of the hop, meaning you can nearly instantly go from the front to the back of a minion wave just by hopping through it, giving a near garenteed escape without having to burn flash, you can cut an enemy off by hoping over them and blocking their path back to their turret, and you can use it to jump over jungle walls, even using certain camps to double hop walls if positioned correctly, and hitting jungle creeps with your wall hop will always put you further then any other form of wall jump or flash, giving you room to escape. it is also one of the best initiations when paired with your mega transformation, especially in team fights where you can hop on top of the front line to get to the back line, and then push them towards your team and lock them down for ridiculous play making potential.

Gnar's R is a great tool, but it has to be used very carefully or it's wasted. passively it increases the speed boost from hyper in mini form, but in mega form it throws all nearby enemies in whichever direction you choose, similarly to lee sin's R, except you don't have to line them up like a game of pinball. it also adds even more damage and a stun at the end if the enemy hit's a wall. with this, you can lock down an entire team by throwing them against a wall, or hopping to the back line to push all of their carries into range of your team, but it's wasted if you only catch 1 opponent with it (unless it's a one on one duel), or if you miss the stun and they escape. be cautious of using it at inopportune times or without proper placement.

Mini Gnar plays similar to most AD carry marksman, particularly the ranged assassins like vayne and teemo, so use your abilities carefully. while he can do ton's of sustained damage rather quickly, he has poor wave clear and is very squishy. poke and initiate on enemies when it's opportune, but be cautious of losing too much health, as you will die very quickly.

Mega Gnar on the other hand Plays similar to tank supports like leona and cho'gath, so using your abilities often is very lucrative. using all of your abilities correctly will allow you to lockdown an enemy under damage from both yourself and your team mates for a very long time, and your greatly increased model and hitbox plus bonus health and armor will allow you to soak up damage, keeping your team mates alive and giving them ample time to do full combos on several targets before they can escape.

you need to play mini and mega Gnar as two completely different champions if you want to utilize their strengths, cover for their failings, and play so their true potential shines through.

"you will only find harmony when you have tamed the beast that dwells within without sacrificing the wide eyed creature that stands before you."