Let me check with my friends ... will get back to you on this.
One says ... Hot and humid with heavy rains.
The other says ... April would be spring time.Any later than that it’ll be monsoon and the wild life sanctuaries would close down. Also if they are planning for Arunachal, will have to check as due to rain, the roads get blocked due to landslides, Shillong is always doable, so are the small towns around Assam.
Hope this helps.