Many of the AOs in the base game are almost ridiculously easy to beat in the final battle. I've been trying to come up with some slightly nastier attacks for them, as follows:

Yig: Each investigator must pass a Speed(+0) check or be Devoured. This check's modifer decreases by 1 each turn (-1 the 2nd turn, -2 the 3rd turn, etc.)

Nyarlathotep: Each investigator must pass a Lore(-1) check or lose 1 Clue token. This check's modifer decreases by 1 each turn  and the number of clue tokens lost increases by 1 each turn (-2 check or lose 2 clue tokens the 2nd turn, etc.). Any investigator with no Clue tokens left at any point during the battle is Devoured.

Hastur: Each investigator must make a Luck(+0) check. Investigators who pass lose 1 Sanity; investigators who fail lose 3 Sanity. This check's modifer decreases by 1 each turn (-1 the 2nd turn, -2 the 3rd turn, etc.).

Shub-Niggurath: Each investigator must pass a Sneak(+0) [1] check or be devoured. The difficulty of this check increases by 1 each turn ( [2] the second turn etc.) Investigators may spend monster trophies as if they represented clue tokens in an attempt to pass this check.

Yog-Sothoth: Each investigator must pass a Will(+0) check or lose 1 gate trophy.  Any investigator with no gate trophies left is Devoured. This check's modifer decreases by 1 each turn (-1 the 2nd turn, -2 the 3rd turn, etc.)

Ithaqua:  Each investigator must make a Fight(+0) check. Investigators who pass lose 1 Stamina; investigators who fail lose 3 Stamina. This check's modifer decreases by 1 each turn (-1 the 2nd turn, -2 the 3rd turn, etc.).

Thoughts? I leave Cthulhu and Azathoth unchanged (Cthulhu was hard enough), and I tweaked Yog-Sothoth just to keep the modifier in line with the others. 

I don't find Final Battles particularly easy once you take into account their modifier and Resistance/Immunities.  But I usually always play with 4 investigators. 

However, you may want to check out Kingsport which adds in teh Epic battle cards.  It makes the final battle less predictable and more dangerous.

I really don't see the need to strengthen AOs beyond what has been done with the Epic Battle cards. There are different challenge levels between them to make each game different. Yig is not meant to be as big of a threat as Cthulhu for example.

 Epic Battle cards are what you need and if there not evil enough for you there may be something wrong with you