Hm, it would probably be better to handle it environmental wise - i.e. instead of making it weaker, make sure the players have a chance to prepare and plan accordingly.. they may have a good idea of the properties of the final adversary revealed from the shadows. For instance they may have glimpses him and his lair through a scrying mirror, or found/liberated a member of the Aboleths minions... Perhaps they are given a talisman which can protect one human from mind-affecting spells (he can then be the character to lure the aboleth into the trap).
You could also make it possible that the encounter takes place in an area where the Aboleth has the upper hand - but through clever tactics it can be lead into a nearby area where it is much more vulnerable.
Don't forget, real world people can kill tigers and elephants - not because we hack away harder or withstand its attacks better.. but because we plan better
Though, the above is really hard to achieve. A player group can sometimes have a bad day, or think that "it's not nearly time for the final encounter", so they don't prepare as much as they should. I have seen player groups walse into tough encounters, but have also seem them overpreparing dramatically for easy encounters.
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