If your main machine, your money-making linux computer, were infected with a very sophisticated rootkit and/or bootkit, how would you go about ridding your device of it?
Boot off Thumb or iPXE and flash the BIOS after confirming with the vendor in writing this is how they would do it for this particular bootkit. If this is not sufficient replace the BIOS chip. If that is not an option write off the server and accept the losses and lessons learned. Implement better security. File lawsuits against the vendor if the server came with the bootkit.