Alright, from what I know, the sight range can be specified in the overworld file, byte 0E to 0F, so I guess you could add your value over there.
If I'm right, that is automatic once you've given the sight range but if I'm wrong then you could do this,
64 00 (NPC ID) 06 00 - Apply movement
1E 00 08 00 00 00 - IDK why this needs to be there
Walk whatever direction, x steps eg. 4C 00 05 00 (Walk up 5 steps)
FE 00 00 00 - End movement
65 00 - Wait for movement
64 00 FF 00 06 00 - Apply movement for hero, that's FF
1E 00 08 00 00 00
Face whatever direction eg. 00 00 01 00 (face up for 1 iteration)
FE 00 00 00
65 00 - Wait for movement
And then your battle script.
For the movement things, check this link
https://pastebin.com/raw/tJXgpmYA