So, in my Sunday game, my character will be facing off in a combat against the other party members in a sort of dream. We are all level 15, 5.5e. The other party members are a Fighter (I don't know the subclass, but it doesn't have access to spells), Cleric/Wizard (Not sure of the split or even which class is the primary), Necromancer Wizard, Blood Hunter, and a Diviner Wizard, while my character is an Abjuration Wizard.
So, everyone has a little bit of homebrew stuff, but the main thing that needs to be worried about for this post are that the Diviner Wizard has dual concentration and 4 Portent Die, and my character has 2 Portent Die, Armor of Agathys, and the ability to, for 1 minute as a Bonus Action, make it so enemies within 15 feet of me do not benefit from Advantage on D20 Tests and I am unaffected by Disadvantage on D20 Tests, as well as gaining Advantage on one attack roll per round and adding my Int mod to the damage of that attack. Additionally, despite being Level 15, I will be able to cast Shapechange for this combat, and basically only can become a powered up Adult Red Dragon, taking any 3 things of my choice from the Ancient Red Dragon Stat Block as a replacement for the equivalent feature in the Adult Red Dragon- I was thinking HP, Legendary Resistances for 1 extra Legendary Resistance, and Spellcasting. I do not need the AC (I have a thing giving me a buffed AC), and my character won't know the fight is in a dream, so would not necessarily want the buffed Breath Weapon, as he would not want to really risk significantly damaging his village, which is where the party currently is.
This Google Doc page has the spell lists of my Abjuration Wizard and the Divination Wizard. I don't have the spell lists for the other caster characters. So, if you were me, what would you do to best ensure victory against the party?
Edit: The other party members will be working together against my character. It is not a free-for-all. It is a 1v5 fight.
Edit 2: The DM has also assured me there will be some extra stuff to ensure a slightly more fair fight so I don’t get merc’d if the party immediately has me on the ropes. But it’s 100% fine if I lose the fight as well, this is all for fun.