In Arkham Asylum, are there Riddler informers

To get the criticals you need to time the button press for your next attack just as the current attack lands. There's a small window around the time so it can be slightly after or before, but not a lot.

Spend some time in the first Challenge room... When you're doing it right, it has a very fluid feel to it. Once you get the rhythm down it's almost hard not to get the criticals.

The moves you have to perform in a single combo are (PS3 / 360 / PC):

• Strike (Square / X / Mouse Left)

• Counter (Triangle / Y / Mouse Right)

• Cape Stun (Circle / B / Mouse Middle)

• Evade (X x2 / A x2 / Space x2)

• Throw (Square + X / A + X / Shift + Mouse Left)

• Takedown (Triangle + Circle / B + Y / Shift + Mouse Right)

• Ground Takedown (R2 + Triangle / RT + Y / Ctrl Left + Mouse Right)

• Batarang (L1 / LT / Q)

• Batclaw (R2 x2 / RT x2 / C)

As far as which order and where to do it, I would check sites like True Achievements (Xbox 360), or PS3 Trophies Forums for some tips. Check YouTube as well, there's loads of videos for it and a bunch of different possible solutions.