Siege Masters...how to get 500?

[Noname]

The last 10% are very hard to get

I actually find that quite manageable, but I assume it depends on what you started with. I, for instance, started by completing the Rag bots first. Now there are only 2 more missing and the rest on Hard anf Normal should be a walk in the park compared to Rag. If you don't need any quick progress, I strongly suggest doing the difficult part (Rag) first, as it can only become easier the further you advance.