This is Trial level 4, "Teach me Digger and Miner". You will have 5 lemmings and must save at least one of them. 10 minutes will be more than enough time. Press X to start the level.
Your lemmings will start trapped in the top area of the screen. Switch to the Miner ability, which looks like a pickaxe, using R1 (for some reason it wasn't selected by default for me on this level.) The darker blocks that have bolts on them are impenetrable, so we want to avoid the lemmings digging into those or they will stop digging, wasting a valuable ability. The Miner ability makes the lemming dig downward, diagonally in the way they were facing. Change one of the lemmings that are walking to the right into a Miner right at the top-right corner of the impenenitrable rock, before the rock starts going diagonally.
The lemming will continue digging until it hits an impenetrable rock or runs out of material to dig through.
This lemming should continue rightward, picking up a new ability.
This ability will make a lemming a Digger. Use L1 to switch to the Digger ability, which looks like a shovel. When a lemming is above no impenetrable rock, use the ability on him.
The Lemming will begin to dig straight downward until it hits impenetrable rock or runs out of material to dig through.
The lemmings will come out into the bottom section of the level.
Now they will continue walking left, right into the goal.