Already doing some long term planning for my second group for this game and I'm going to want a Ranger.

I read somewhere that they can instant kill with ranged attacks, but can't even find that source again. I'm confident this is true because I already got a character killed by a Gorn with an instant arrow kill.

I don't know how it works though; Rangers don't have Kirijutsu. Is it just a base 1 or 2% chance per ranged attack or do they have to pick up Kirijutsu through class switching? And is it all ranged weapons or just bows and crossbows?

Critical percentage is based on the level of your ranged combat skill. The percentage change is around 1% change for every 20 points of skill. Please note that weapon critical kill percentage is added onto this, so if you had 100 ranged combat skill and a weapon that added 5% then you would have a 10% as they add together.

To have a chance to roll a critical hit you have to actually hit, penetrate and do at least 1 point of damage.

The part that really stinks is then the creature you hit then has a chance to resist the critical hit and that is based upon a level comparison, so if you are equal then they have like a 50% chance to resist. If they are 1 level higher than you then they have a 55% chance to resist and if 1 level lower it will be 45% chance to resist(maximum 95%). Please note that most boss monsters are immune to critical hits.

Cosmic Forge names "Missile expert: Kia x 2, Miss chance -15" as class features.

Some launchers or ammo grant a chance for critical hits and I guess being a ranger doubles this, they don't get a genuine chance to score criticals, only +15 to hit with missiles.