I'll try and do as many as I can, though some remain mysterious to me considering I haven't seen all the movies (KG seen in movies are a bit unrealistic IMO).
Rain
Weak to: Earth, same as normal water.
Lava
Weak to: Strong lightning (as seen in the latest chapter) or strong water. Preferably a combination of both.
Wood
Weak to: Lightning and fire. Lightning was seen to be quite effective against wood (look here:
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Crystal
Weak to: There's a chapter on it on Narutowiki
"The Crystal Release, though powerful, has two known weaknesses, the first being that it cannot crystallise either raw energy or chakra as they have no physical mass to crystallise, allowing a person with either energy-based techniques, such lightning, fire, wind, or good chakra control to properly counter it. The second weakness of Crystal Release is that it is susceptible to a specific sound wave frequency; because crystal requires a very precise build-up of molecules, the proper sound waves can break down the build-up before the crystal can properly form. Rinji, taking advantage of this weakness, used his bats to counter Guren's Crystal Release."
Storm
Weak to: Considering it's basically lightning infused in water to direct it at the opponent, a strong wind technique could be very effective. You could also use a large water source to diffuse the lightning, leaving nothing but a water jet.
Ice
Weak to: This one is pretty obvious; fire and lightning combinations (very strong fire though). Everything that is strong enough to shatter the ice could be useful too (earth or wind to cut it).
Dark
Weak to: This is one of those element which I'll have to watch the movie to give a clear answer. It seems they don't even know what elements are at the rout of Dark release.
Swift
Weak to: Same as above, the nature of the chakras are unknown. I'll have to watch the Hiruko movie I guess...
Steel
Weak to: Though it is not stated which elements are used to create the this one, it's obvious what is strong against it. Lightning will travel easily in it, making it useful if it's connected to the one using it. Intense flames could possibly melt/soften it (1500 degrees celcius to melt it). And if you create large source of water to stand on, the steel will sink.
Boil
Weak to: Official description;
"that combine fire- and water-based chakra to create a powerful corrosive vapour capable of melting anything."
Use Lightning or Wind in the appropriate situation. Lightning can be channeled through the vapour and cannot be melted. Wind can blast the vapour away or cut through the mist (depending on the Boil technique used, be careful and read the descriptions carefully).
Scortch
Weak to: Too little was seen of this element, I will not make a prognostic on it but here's an advice; since it dehydrates you to the point of death, surround yourself with lots of water to block your opponent using this KG (same as Luffy vs Crocodile for those of you reading one piece).
Sage Jutsu
Weak to: This is basically chakra, so defend with techniques that counter the effect of the jutsu used against you with any appropriate element.
Example: someone shoot a blast of condensed Senjutsu chakra to me (A-rank), I'd defend using a large A-rank/S-rank earth technique or a very large body of water to stop it dead.
Shadow
Weak to: Light. So Fire is good to repel shadows (though you create other shadows around you), lightning can be used to create bright flashes, annihilating any shadow around you for a brief moment.
Bugs
Weak to:Anything that could kill them really. Squash them with Earth techniques, drown them with Water techniques, burn them with Fire, fry them with Lightning, or blow them away/slice them with Wind. To fight a bug user it's mostly about timing and not letting yourself get surround (they'll suck your chakra dry).
Steam
Weak to: Large Wind techniques to blow the steam away, Earth defenses or attack are too dense for the steam to have any real impact (unless stated in the description; steam spike for example, but they'd still be weak to earth). Steam jutsu are generally kept are normal pressure, meaning they can't be heated that much (not close to strong fire techniques), so it's very unlikely they'll turn solid rock to lava (unless stated in the jutsu description).
Blast
Weak to: Still too little information on this KG. However, Deidera was considered a wielder of this KG according to Narutowikia, so it's safe to assume is it an Earth related element, meaning lightning should do the trick against it.
Sound
Weak to: Wind, nuff said. If you're quick enough to pull it out, shoot massive wind blast and you can repel most sound techniques (since the air is the medium for sound). You can also protect yourself by placing other mediums between you and the sound blast (if it's direct and not based on your sense of hearing), like Earth walls and Water walls. Be careful though cause it could very well backfire on you if it's use in the wrong situation. For inexperience RPers, keep yourself to Wind techniques.
Magnet
Weak to: This is a hard one since it was never really established how it works in the Narutoverse. I will go with this; Magnetic release won't affect you directly, the user of this KG needs to attack you with Gold/Iron or ninja tools, so just stick with tangible defense. Earth walls, strong Water defenses...etc. You could also use the fact that Lightning can be channeled in Iron/Gold to hit your opponent if he's in direct contact with it.
Sand
Weak to: Same as above, though for this one there's another element you can use; Wind. Since Sand is lighter then Gold and Iron, you can use the according wind technique to attack/block Sand release jutsus.
I think that's it O__O
