Boulder Dash-XL

While the desciption says Arcade Mode it it possible to get this in retro mode. The first amoeba is on level 5 (cave E) in retro mode.
Amoebas are patches of green slime that keep expanding, but they cannot expand through walls, rocks, diamonds, or you. They only expand horizontally and vertically. When they can't expand anymore, at which time they're considered entrapped, and they turn into diamonds unless they've grown too large in which case they turn into rocks.

The achievement popped for me after the amoeba turned into diamonds. I dropped the two diamonds above me at the start onto the ameoba to block it's upward path. Then i headed right to drop and push a couple of rocks into the the two small paths on that side. Now its over the the left whey you need to block a path left of center before heading over the the far left to drop the rock and diamond to block the final path for the amoeboa, possibly standing in the last gap.

EDIT:

The first amoeba in arcade mode is in Cave 47. The ameoba starts in two area on both sides of the start/exit area. What you want to do to trap this on is to leave a line of dirt about the two enclosures with gaps above the walls except the left walk of the right enclosure. The drop rocks so that there is a rock on each wall and a line of 6 rocks over both enclosures. Stand in the gap over the last wall and wait for the amoeba to turn to diamond. Use the grapple in the center to pull most of the rocks to the center so you can safely scoop up the diamonds.