Obligatory LBG infodump copypasta.

3U primer. If you want to skip this in-depth crap just watch this.

LBGs are elementally-focused weapons. Generally you want one gun per element, and you want to make sure that gun has a decent Normal 2 clip size to fall back on.

• Normal 1 does 6% of your display raw* in damage. Never use Normal 1.
• Normal 2 does 12% of your display raw in damage
• Normal 3 does 10% of your display raw in damage, +10% per ricochet hit up to 3 additional hits
• Pierce 1 does 10% x3 hits
• Pierce 2 does 9% x4 hits
• Pierce 3 does 8% x5 hits
• Critical Distance (being within a certain distance bracket from the monster) boosts your damage with all these raw shots by 1.5x. The bracket is different for each shot, so it'll take a while to get a feel for. See here (you should watch that whole thing at some point).
• Elemental shots do 45% of your true raw* in elemental damage, plus 7% of display raw in damage, regardless of distance as long as the bullet doesn't fizz out. You'll know you're hitting if you see an elemental hitspark.
  
• Rapid Fire modifies each of these shots differently, but generally it's [.7 x 3 shots] per volley. For volleys of 4x Normal 2 (before Bonus Shot), it's [.6 x 4 shots].

^{* Display Raw and True Raw - See here for scary math}

It's extremely important that you bring the right bowgun for the monster. Get familiar with kiranico and do your research before taking them on. Match elemental weaknesses and make a note of high-damage hitzones. Where you shoot the monster is super important for achieving good, and often superior, kill times.

So what makes a good bowgun? The ability to rapid-fire an element is mandatory*. An Average or faster reload is desirable. Deviation is manageable as long as it goes in only one direction; avoid guns with L/R deviation because your shots will essentially land in a random cone.

For your early armory you're going to want Royal Torrent (Royal Ludroth) for Water, Khezu Syringe (Khezu) for Thunder, and the Rathling Gun (Rathalos) for Fire (builds from the Cross Bowgun -> Grenade Launcher line). Unfortunately you don't get a good Ice gun (i.e. no L/R Deviation) until High Rank with the Blizzard Cannon (Barioth) or the Frigid Bowgun (Kirin). The Khezu Syringe loads Ice S in the meantime and has a decent Normal 2 clip.
In High rank you'll want to replace the Khezu line with either of the Zinogre lines for Thunder; the 5-shot rapid and average recoil on Khezu's and Lagiacrus' later upgrades make the guns unmanageable. Kushala's LBG may also make a good Ice replacement in G-rank.

*^{Some bowguns like God's Archipelago and the Felyne Helldoll specialize in delivering status ammo and playing utility roles. You won't be using them solo unless you have a very specific plan in mind.}

Priority Skills

• Bonus Shot (not available till HR7)
• Evasion +1

Offensive Skills

• <Element> Atk +1/2/3
• Elemental
• Normal/Pierce Up (on certain bowguns)

Defensive Skills

• Evade Distance (nice, but unnecessary)
• Evasion +2/3

Utility Skills

• Combo Plus
• Recoil Down +1 (on certain bowguns)
• Precision (on certain bowguns)

Armor

• Offensive spec (Low Rank) - Velocidrome
• Defensive spec (Low Rank) - Tetsucabra
• Evasive spec (Low Rank) - Lobster or Narga

• Offensive spec (High Rank) - Velocidrome S or Jaggi S (gemmed for Attack)
  

> * Upgrade to Zinogre U later with gem/talisman sets for each element

• Defensive spec (High Rank) - Tetsucabra S
  
• Evasive spec (High Rank) - Lagombi S

When you get to G-rank your options open up immensely, but remember this: The one skill you absolutely need on any set you make in G-rank is Bonus Shot. For most elemental LBGs it's an ammo efficiency increase of 33% by itself.

Q&A

>good G2 light bowgun

You want a stable; 1 for each element. The gun lines mentioned above upgrade very well and remain competitive even in late G-rank, but do feel free to explore a little.