Well biel-tan are known for having large numbers of aspect warriors. A more fun alternative would be to make your own craftworld explaining that they like warp spiders.
Secondly notice the warp spiders are good at taking down light infantry and very light vehicles. Anything else they struggle due to lack of AP. I would recommend having pathfinders as troops since they can take care of heavily armored stuff as well as dark reapers for killing the ever so common marines. Perhaps having a dedicated anti tank unit like fire dragons in a bright lance serpent would be good.
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