GBT's shot at a No-Vet 500 list: (Note, that this is an excercise, not a recomendation. This is hard at 500 pts.
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CDX - Primaris [Weird Boy]
CDX - PCS: [Gretchin Minderz]
70 - Infantry Squad: 1x GL, 1x Missile Launcher
70 - Infantry Squad: 1x GL, 1x Missile Launcher
105 - PCS:
Captain Al [Gretchin Captain], 1x Flamer
155 - Infantry Squad [20]: 1x Commissar [Runtherd], 2x Meltabombs, 2x Grenade Launcher
So this Weirdboy goes right off the deep end, sees himself as a Gretchin Messiah, convinces a Runtherd to bring his flock along to go sew discord and meyhem through the galaxy... The Gretchin leader finds himself a sparkly stick o' doom, and taa-daaa!
This, thematically, avoids energy weapons in favour of explosives and flamers, which the Orks tend to use instead. Replace "Meltabombs" with "Tankbusta Bombs" and you're good to go!