Alright then z.z, lets start with earth+fire.
Now there is a lot that can be done with this and to be honest, my sensei Negative Knight, his favorite elements were fire and earth to use in battle and combining them. So I guess I have a pretty good knowledge on this subject. When fire makes contact with earth, there is two things that can happen. One, the earth structure will be super heated, obtaining a burning property to it. The next is, the subject can melt turning into a pseudo-lava. Now lets examine the effects for each.
A super heated object, specifically an earth object, now their sustainability is decreased dramatically, because sooner or later the heat will force the shape of the structure to deform and die out, this is specifically the case with earth techniques that require specific shapes and forms. However, some more then others. If touched, obviously you will be burned, but imagine using this on a huge colossal being? The being not only is super heated, but now it is subjected to have a brute force impact due to the natural ability earth grants it. Its hard enough to have a huge fist or ball of earth coming at you, but now imagine it being super heated. Different ball game. So as you notice this is quite advance.
Now with the same thing, this time to the point where the subject is melted and turned in a lava-like substance. Now this is restricted, because not all earth will allow this, however its best used with techniques where there is no specific shape of the earth that forces the technique to have for it to sustain itself. So this can be used on spikes, pillars, balls, etc. Nothing with a specific shape. Things like Golems, animal like shapes, etc..might not be the best bet to use this with. Also, this process can be sped up might faster if you use the earth alternative form, mud, since it is already a liquid substance of earth, super heating it is much better. Now a lot can be done with this, binding an opponent with the liquid form of mud and super heating it can be a 1-2 knock out. Evasion is difficult and the effects of mud is enhanced and more lethal. Tactics is higher in use with this form of combo.
However, something is key when using this form of combination. If you use earth techniques and combine it with fire, if you want the earth to remain solid but super heated the earth technique must outweigh the ranking of the fire. So using B-Rank earth along with a C-Rank fire. If you want pseudo lava, then it would be the reverse. A C-Rank earth used with B-Rank fire would crease the lava like substance. If you combine both with same rank, you can decide which one, there is no clear cut form, but there will likely be no argument from either end. So go with what your gut tells you when combining same rank.
BOY, that was a lot of writing. Anyway, I hope you read all of that. For review, here are some questions I'd like you to answer, so I can tell if you read all that or not.
1. What fire rank is best used to super heat a B-Rank earth technique?
2. What fire rank is best used to make a B-Rank earth technique turn to psuedo-lava?
3. What elements would be strong against a fire/earth combo?
4. What elements would be weak against a fire/earth combo?
5. What elements would be neutral against a fire/earth combo?
(Use basic knowledge and logic for 3-5)
Simple questions z.z