Its such an open ended question its hard to reply to.
What i can say to help is:
1 look into HLOD use for buildings
2 use lod groups, for everything you can.
3 use the foliage tool and not staric meshs if you can as each foliage tyoe is only one 'draw call ’ ( look those up if you dont know).
4 AVOID the grass tool,as its the devil itself, till a hoped fix in 4.20.
5 use foliage tool for grass, and if possible turn down shadow use
6 try to keep landscape component total low
Thats it! Hopefull may help others as unreal 4 docs are awful for all the granular detail things that matter