"During a battle, you may spend 1 charge from this card to treat one of your enemy's dice as a 1 result."

Can I use this charge immediately after enemy battle roll?

or is it supposed to be used instead rolling enemy die?

Yes, after the dice were rolled.

Ah, that's good to know. It does say "during a battle", which to me wasn't precise enough since there are so many steps to a battle. I always played it as a wargear, i.e. you have decide to use during it your prepare battle step before the ennemy rolls a die.

But you are supposed to use Charge Tokens and choose a Weapon Asset before the enemy rolls so I do not see how you could use it AFTER the enemy has made their roll.

The only weapons that are applied before the enemy rolls, are those that give direct bonuses to your battle score.

Any asset that is used for its reroll ability can be selected at a later stage.

See page 12 of the rulebook under section 2: Prepare for Battle and section 5: Determine Battle Scores.

I would say that a weapon that affects an enemy's die roll can be applied at stage 5.

You're still limited to only one weapon per battle. If you have chosen the Storm Bolter during the Prepare Battle stage, you cannot choose the Web Gun at a later stage of the same battle.

(Unless you have a corruption card that lets you use 2 weapons at once)

Good point but in the example following the section you are referring to the Active Player discards his Charge Token before the enemy rolls. Also, in the fifth section the phrase "any card abilities" seem to be referring to the Enemy Threat Card only as it is followed by an example where the enemy adds 1 to his battle score for each Power Card the Active Player has.

In the case of the Web Gun this would mean that the enemy need not roll the die as it is done for him by way of the Web Gun's effect but it still happens after he has spent his Charge Token.

In short you should use the Web Gun BEFORE the Enemy makes his roll.

Okay, I've found the answer!

Read the Timing Language box on page 19.

The keyword is "during" .

Players can resolve abilities that use the word " during" at any time (of the player's choice) or step to which "during" is referring.

The Web's Gun ability calls for discarding a charge token to change the enemy's die roll to 1. Since the card's description contains the word "during", it can be activated at any time, that is after the Enemy has rolled the dice.