It all boils down to your personal luck. How often can you Death Adamantoises and how often do they drop Platinum Ingots for you.
Personally, for me, it is very unpratical. Not because the Death part, I'm over that, but because I have crappy luck in the drop % of Ingots.
I think it's best if you just try to get Trapezahedrons out of the Adamantoises.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
