Receiving “connection with batcomputer jammed” when trying to disarm jammer

Here's a tried-and-true strategy that will help you rack up high combos full of criticals and not get hit.

First of all, you need to find a large group of thugs to fight. You can't get high combos if there aren't enough thugs and you knock them all out too soon. You'll need to be able to deal 23 attacks at a minimum, 3 regular hits since criticals don't seem to count until you reach special combo status (the next hit after the initial 3 can start counting criticals) followed by the 20 criticals. The more there is, the easier this will be. Try to find a crime in progress, at least these encounters, there generally are less guns you have to deal with.

The secret to this challenge... dodging.

That's right, build up your combo to reach special combo status, then dodge and hit, dodge and hit and repeat that pattern until you get the required amount of hits. Dodging pretty much guarantees you won't get hit so you're safe from attacks. And when you attack after landing, the game counts it as a critical. If you build up enough charge on your shock gloves, criticals will increase your combo multiplier by 3 building up your multiplier even faster.

Just note that while this strategy helps build up your combo, it alone doesn't help much in the challenge maps, you can't build up a lot of points doing this. You need to mix in different gadgets for variety and takedowns to take advantage of that multiplier.

This video describes how to beat this particular attack. Apparently it can simply be countered. The first "attack" can also be countered, with the option of recovering instead (grappling back onto Firefly).

Jeff's comment links to a non-NG+ video that has the cues for the counters; the same countering will win in NG+, but you don't get the nice on-screen cues.