I thought a guide for people just trying to earn this at the beginning of the game would be useful. After getting Solas and Varric in your party, you’ll start up a mountain. Select Solas. Everyone will now follow him. When you find some enemies, hit
to enter tactical mode. Select the closest enemy with the cursor, and hit
to cast Winter’s Grasp. Switch to Varric. Hit
to use Long Shot on the same enemy. Now advance time several steps with
while still controlling Varric. You should see Solas cast, the enemy freeze, then Varric shoot, and the achievement pop.
Here’s a full list of possible combinations for people later in the game:
https://dragonage.fandom.com/wiki/Cross-class_combo_(Inquisi...)
You need to use an Ability that Triggers Status, and then an Ability that triggers Detonation. Tactical mode is definitely the most consistent way to do this.