You MUST use at least 20-25 (50-100 for northmen) supporting troops to gain FULL benefit of Legendary troops. Otherwise the death rate of legendaries rises dramatically.. even if you are ONE troop short. Best to use a buffer (personally if i have 10 Consumed Ones, i pair them with 300 King Elf Mages - instead of 200).
I dont recommend legendary troops for OBs as the defense bonus caps out at 25% (and NEVER use them in stashes) - best to use them at home in your castle where they REALLY can make a huge difference in battle. The exception to this rule is Angernauts (legendary scout) are great to park in OBs (pair up with 20+ scouts) to prevent OB scouting attempts.
Finally NEVER use legendary troops in BGs unless you are stuck.