I see a lot people having struggle with that, so here is a quick and easy solution I wanted to share.
Step 1:
There is this tile in Dungeon_A2 - cut it out.
Step 2:
Duplicate it two times and merge the 3 layers.
Step 3:
and
Add your two terrains as seperate layers.
Step 4:
Add a white layer mask to the layer you want to be in the on the "outside".
Step 5:
Copy and paste the modified Dungeon_A2 tile on the the layer mask.
Step 6:
Merge the result and copy it on your A2-Sheet.
Step 7:
Be happy, you have smoother transistions between your terrains now, works for most "natural" terrain types.
Step 1:

There is this tile in Dungeon_A2 - cut it out.
Step 2:

Duplicate it two times and merge the 3 layers.
Step 3:


Add your two terrains as seperate layers.
Step 4:

Add a white layer mask to the layer you want to be in the on the "outside".
Step 5:

Copy and paste the modified Dungeon_A2 tile on the the layer mask.
Step 6:

Merge the result and copy it on your A2-Sheet.
Step 7:
Be happy, you have smoother transistions between your terrains now, works for most "natural" terrain types.


