You want to pit your Lillpup against a Gastly? Man, you're more gimmicky than I am. Unfortunately, Lillpup in LC has little to use in hurting Gastly a whole lot. We can try to find something though.
Return should always be the STAB option on a Normal-type. It's an awesome move!
For Ghost-types, the best you'll get for LC is probably gonna be Pursuit, which is pretty useful, since scoring a KO on Gastly will be easy, since he'l have to stay in for a 2HKO, or switch out for a OHKO. Odor Sleuth is also an option to set Gastly up for an unforgiving Return.
Wild Charge may have recoil, but the physical Electric coverage is gonna help a lot. If you want something less risky, go with Thunder Fang.
The last slot has a lot of options. Yawn can put things to sleep, or can force switches to set up for Pursuit. Fire Fang can rid of Ferroseed and other Steel or Grass-types. Ice Fang rids of Dragons, mostly. Work Up or Howl could be used as boosters, as well.