From the mechanical point of view, the most significant concern for solo monsters is status effect protection. There have been a number of different ways to solve this issue. Take a look at dragons from MV with their Action Recovery and Instinctive ... traits for one method. You could also give Allabar a minor action "get rid of one status effect by taking 15 damage" or "transfer one status effect to an ally within 10". If you're feeling evil, and you should, let this power consider creatures sprouting tentacles (from the Wrath of Forsaken World power) as allies. Which ends up being very close to some of the abilities I gave to my own Big Boss monstrosity, but that worked rather well, so why not.
Yet another method of protecting solos from nasty players is to downgrade any status effect inflicted upon them. I've described it here and it works for me so far:
When a Solo gets subjected to any of these effects, simply downgrade it by one step. So a power that makes its target stunned would only daze a Solo. This is the core, reasonably safe option. It should solve most of the Solo problems, significantly reducing their need to rely on exception-based status effect avoidance.
Specifically for Allabar and your party, consider having Psychic Boost recharge upon being critically hit instead of being dealt psychic damage. Unravel Essence is nice and scary, but won't actually do much as it is written. Perhaps moving its effect to Psychic Boost instead of its damage would help, with the inflicted condition worsening if the creature is already suffering from one.
Spawning minions (or regular enemies with halved hit points) to occupy the AoE-happy party members each round may be of help.
In addition to the mechanical side of things, consider the flavour. Avoid the final fight occurring in a featureless room 10x10 squares. Make it big, flashy, exploding. Without knowing how Allabar is involved and any surroundings, I can only give out random suggestions here. What if the fight happened mid-air while flying over the world towards a city important to the players, with the effects of Allabar's gravity gradually pulling more and more of the land below as it strengthens? Random attacks against the combatants because a house flew up and hit them are fun. And if the fight isn't over in a given number of rounds, the city itself gets sucked up. High stakes!
Change things up in the middle of the fight. To avoid having this answer be completely self-referential, here's a link At-Will's Worldbreakers. The example of the Etherkai should be of particular interest.
In the literally world-breaking example above, some structure important to the campaign, like a temple of a PC's god could be sucked up at round 4 or so, briefly breaking up the fight and changing the landscape.
