Option 1: Increase their HP
Option 2: Make them into Fates like Ixion, etc

The current situation isn't fun. Hitting the Hunt mark 1 time and waiting for others to tag it to get credit is the definition of laughable content.

The fact that you've gone 8+ years like this without addressing it is a bad look. This isn't FFXII where you and your party are the only one who will discover hunt marks, this is an mmorpg, start treating it like it's actually one.