Have no fear your Biel-Tan Young King is here.
Well by Pirate army fo you mean DE or just Craftworld Raiders that you made up for yourself?
Anyhow the best bet is go with the idea of a Boarding team. You know the kind that is used in ship capture/raids. Since the Craftworld Equipment tends to be superior to DE or Pirate counterparts and has better stats overall you can go for a specialized boarding unit. Trained in surgical strikes vs ship systems and operations that is often used in Land engaments because of their specialization in the area and the fact that Aspects are not really available to Pirates all the time are they?
Want more ideas? Give me something to work with...