I recommend getting a Skullgreymon, erase it, convert a Botamon, raise it to Machinedramon(Need to have had MetalSeadramon(I don't know why)), get it to have 99 aptitude, get it to Lv.99, DNA digivolve it with a Kimeramon, raise the resulting Milleniummon to Lv.99, and do a trade-back with it.