Some input from SE Asia:
The knife fish/featherback has at least 4 species, as far as I know. There is this small one (Notopterus notopterus) which doesnt grow much bigger than your palm. Then there's the Notopterus chitala, which you are holding in the pic (I think!). In Malaysia, this grows to 27 kg (about 60-lb), although 3 t0 7 kg is the norm. The other species are the spotted knife fish (Thailand) and the beautiful striped knifefish (Cambodia, I heard).
The knfefish are predators, feeding mainly on small fish and prawns. They swim at a 45 deg angle, using their long pelvic fin as a tail. Bottom feeders, mainly. Their mouths are relatively small, so baits have to be small too. Use local prawns or baitfish. A No. 2 or 1 circle hook will serve well, ensuring hookup at the corner of the mouth. Allow the fish to run off with the bait before tightening up. I use a leader of 20-lb braided, green colour, just in case.
In my experience, knifefish feed best on calm days (no winds), at dawn or sundown. Once the sun dips below the 10 deg angle, the sudden darkness in the water brings them out to feed.
Lures need to swim close to the bottom, and swim slow; knifefish are not speedsters, although they jump on hook up.
Hope this helps.