We just finished Book 4 last night. Here is what I did to make the final fight a bit more challenging:
My players are level 13 mythic tier 1 - I have a wizard, cavalier, slayer, hunter (favored enemy giant of course), and a oracle of life follower (level 10 as a follower - the cavalier has leadership).
I used the mythic version of Mokmurian found in this forum under 'gm's guide to challenging fights' - with one exception - I gave him the dual initiative ability.
The party skipped 1/2 of the upper halls - and wiped out all of the lower halls but had decided to avoid the library until they explored the rest of the corner - and then met Big M - so they did not have the shining child fight.
They entered the fight with Big M down fairly well on resources - the last time they had rested was with Cona watching over them on the upper level - and they tackled the lower area without stopping. Big M had spied on them throughout the level - they even caught him doing it in the headless lord's gallery - the wizard was using detect magic and I let him see the divination effect on top of the headless 'statue' before they came to life.
So they met Big M with him being very prepped - the entered his hall (which btw - I needed 2 maps for) and saw a giant in robes with their back to them in the middle of the hall. Imitative and they hear a spell - solid fog right in front of them. They move around the fog cloud - they see the giant move 'oddly'. Cloudkill pours from above the ranger. Acid fog on top of the cavalier.
They move around the fogs and the ranger shoots the robed giant - the cavalier moves around the giant to charge and sees that it is Cona - tied up and made to look 'wizardy'.
The slayer moves up and frees Cona who shouts 'he's above you fools' (note 100 foot ceilings are dark and out of sight even though the hall is lit by continual light spells - they have a 30 foot through so the ceiling is in a shroud).
Cona casts fly on the slayer - black tentacles take out the ranger and oracle and wizard - the wizard uses a school power to teleport the oracle and himself out of the tentacles - the ranger struggles. The cavalier feeds a potion of fly to his dog and joins the air battle.
The fight went pretty predictable here - until the wizard said 'bring it' resulting in a failed save for the cavalier against disintegrate - and a failed save for the wizard against baelful polymorph - resulting in a rabbit (the wizard kept his mind intact however - and the slayer could understand him due to his helm of comprehend languages). The cavalier was nothing but a pile of dust - another disintegrate was almost failed for the slayer - however a mythic surge saved the day - and the players were able to finish off the 38 hit points Big M had left - resulting in a big victory (after the oracle used the scroll of true resurrection found in the black monk scroll).
We now take a brake as the wizard takes over GM duties for our wrath of the righteous game - and I prep for book 5 - the library will be played via email between games using the research system.
My players are now level 14 tier 1 - I plan to bring them to level 20 tier 3 by the end.