Darius, as I have demonstrated earlier, you will reap greater benefits from Bahamut's HP +50% the higher the levels are, meaning it is more beneficial to do it at the last few levels.

In your example you said to use bahamut from 50-90 in order to get 9999 HP, and then equip espers that boost other stats. In my example, you can spend all the levels from 50-90 boosting stats, and then equip Bahamut for the ten last levels. That way will still get you 9999 HP, but you will in addition have a lot higher stats.

And good luck beating an intangir in WoB without vanish/doom. However, encountering it and fleeing the battle is enough for it to appear on the Veldt.