Clash interactions in DFCI are at most a 3 tiered interaction, starting from AB moves and ending in a number of possible options. They are a common occurrence so understanding the options to counter them will enable you to turn the opponent's reversal attempt into their own demise.
All AB related moves and certain character’s special attacks have some form of guard point (Impact Skills being from frame 1-8 and Impact Break being frame 11-21). If the player is hit during any of the these frames a clash effect will appear increasing the hitstop and allowing the opponent to counter with a move of their own.
The amount of hitstop that occurs during a clash is dependent on the strength of the attack that triggers the guard point. A attacks have a small input window resulting in less reactionary time for the opponent while B and C attacks have a larger input window. When countering with a move the rules of cancelability applies the same so depending on the move that triggered the clash, the moves you can counter with will vary. For example clashing with a special move will limit your choices to anything that is cancelable from a special attack, so Impact Skills will be unusable.
Now that we've gone over how clashes occur we'll go into how to deal with them. Starting from the most basic interaction, using your own Impact Skill to counter the opponent's guard point move is the most common way of countering clashes. It costs no meter and in most cases will grant you a knockdown and start your turn. However this is also the most vulnerable option as using your own guard point frames to clash against the opponent's attack opens up the same opportunity to them allowing for another clash interaction. Depending on the resource situation of the opponent this may be a good way of forcing them to spend resources to make themselves safe.
As mentioned in the example above depending on what move triggered the guard point you might be unable to counter with Impact Skills, in this case the next resource option are EX moves. In most cases EX moves will get hit but in certain cases for example Mikoto or Asuna 214AB can avoid some attacks due to their quick backwards movement, or Kirino 236AB which is the only fully invulnerable EX move in the game.
For Climax Arts because 41236BC does not have any invulnerability it is ill advised to use in any situation. 63214BC on the other hand is the most common option when shutting down any further clash interactions as it is completely invulnerable, not allowing the opponent to clash with their own move. However due to the input motion and limited input window it can also be the hardest to do on reaction.
The next step up in resources is Combo Blast. This is universally the best option as it works in any scenario and can result in a damaging combo as well. Because it requires your blast gauge it is also a very expensive option and should be considered depending on when the clash occurs. Do you have other options available to counter the clash like Impact Skill or Ranbu? Will this kill the opponent with the following combo? If I don't counter the clash how much damage will I take? These are factors to be aware of before spending Combo Blast to counter clashes.
Equally or even important in the resource hierarchy is Trump. While this can only be performed against grounded clash situations it is a good way to use Trump if you have the leeway to use it. However because it is such an important resource careful thought like using Combo Blast should be considered before using it.
Aside Combo Blast all other options when clashing with a jump in attack are unavailable. However the one caveat is that if the player has not used their double jump prior to the clash it can be used to jump cancel during the clash interaction and block the incoming attack. This is the safest way to deal with clashes when in the air but is limited to your jump usage and only if a normal attack triggered the guard point. As for why this does not work against grounded situations, while you can technically jump cancel a clash interaction because of the jump startup frames you cannot block by the time the attack comes out.