Knuckle Cracker

With max build-speed upgrades, it is in general possible to snipe a Sleeper node with two Guppies and a Nullifier, so long as there is not creeper on the ground around it, either because of nearby weapons or because it has just landed. The Nullifier will finish before the first mortar shot from the Sleeper lands, and it charges fast enough to fire before it dies to subsequent mortar shots. Technically only one Guppy is required—the second one is insurance.

If you clear enough of the map to build sufficient energy and weapons you can rush the inhibitor clear up some mess and take out a sleeper.

It's possible to actually snipe the Sleeper immediately when it spawns. You need max build speed, and some packet speed may make a critical difference too—I had +10. Get enough weapons in place to suppress the Inhibitor, and build a nullifier, but leave it disarmed until you're sure you're ready. Land two disarmed Guppies just outside the ring of terrain around the Inhibitor. Arm the nullifier and kill the Inhibitor. Single-step the last few frames of the Sleeper landing animation, and immediately when it lands, arm the guppies and build a nullifier on the high terrain around it. The Sleeper will fire at it—but it will finish before the mortar round lands! And for whatever reason, the Sleeper's mortar does very little damage to Nullifiers, so it will survive long enough to fire.

Build two Nullifiers next to the Inhibitor, and only arm one of them before the Sleeper lands.
I'm guessing Decagon locked that one down.

The Sleeper destroys all weapons...I think on the entire island, certainly those inside that ring of terrain around it...when it lands. I tried the second-Nullifier trick myself, and then when that didn't work, wondered if it was just Nullifiers and tried filling the entire island with Snipers. Guppies are unaffected, though—and if they were, you could probably still do it with still-in-flight Guppies and perfect timing.

Build one bertha on a power zone, and then target any sleeper node. Disarm your Anti-creeper bertha, let it build to 100, and then wait for your power zone bertha to fire. Arm your AC bertha a few seconds after the 3 shells pass overhead and it should land shortly after the bertha shot hits. This will kill any sleeper node in about 3 hits. Daniels' solution is better if you're paying attention though.  ;)