Exactly. Even if you had life aura 3 with holy panel (sanctuary would be better), you still wouldnt be invincible. Bass v3 could still break through it (I think).
I havent played Megaman Battle Network 2 in a long time, so I dont know if your aura fades after a while or not. Still, there are chips that could stop that trick anyway. Anything that changes panel types can remove sancuary or Holy panel can reduce the amount of damage you can take to less than 200. And poison panels can still hurt you even if you have an aura.
To call yourself invincible, you would have to be able to change panels to holy panels at will and have a life aura 8 or something. (I dont know if Gospel has an attack that does 400...).
Still, in Megaman Battle Network 2, that combo is almost invincible because 400 damage is almost half of your life, where in Megaman Battle Network 3 and beyond, you can have alot more, somewhere near 3000 (using lots of programs). There is a code on the mod tools that gives you +1000 Hp, but at a cost: you must not have ANY errors in the navicust (you must enter the +1000 code in the error screen) and any panels you walk on turn to poison. There are weaker versions of the +HP codes, like +250, but any higher than 250 and you will have bugs!
But, there are other methods of "almost invincibility". I only know one other one, but it does awsome, no matter what the damage! Ill put it in spoilers, just in case.
Spoiler (click to toggle)
Have a wood style and have grass stage as your pre-set (even better, have it as a navi-cust program). Also, have Under Shirt on so you will be left at 1Hp no matter what (as long as you came from above 1HP). Now, when you get hit, you could be left at 1hp, but the grass stage heals you to above 1hp. Then when you get hit again, you will still be left at 1hp. You probably wont survive rapid attacks such as the guts style machine buster or the twisters, but anything else will keep you alive.
The same rule applies to this as the other "almost invincible" trick, a change in panels, and you have a problem.