Tristitia is easily soloable. Ducal/MM/Beyond and Geocrush every time it's up. Now every SMN can get +2 pants :D
I subbed NIN so as to not get wiped out by Thunder V on pop and used Scherzo-pact to make sure Thundaga IV didn't take me out on pop. Run it to flux 0 and fight it there. One Titan should last ages especially if you keep reapplying Stoneskin to him. When you need to resummon, let it cast something, THEN run and dismiss, so its spells will still be on cooldown and it won't run over and oneshot you with Thundaga IV (which is the only realy danger here. It hits 1.9k+)
What VNM path do you take for T1 and T2 to pop him frequently enough to solo all of your Cards? And about how long does it take you to get that T3 Abyssite? I'm extremely interested in doing this solo whenever I've got some downtime due to the fact that I need the Cards for my three main jobs.
I usually change my T1s in Konschtat, just cause its easy :P
The T2 in Misareaux is a total whore but it's the only way to get Tristitia's pop. It casts every element of -aga IVs, Hexidiscs can hit up to 1.5k (on you, not the avatar), Actinic Burst gives it a potent Enlight and several shadows. It has 100% DA and hits HARD.
Regardless you can still do it. Ducal/Mounted/RR, pop it and Pred Claws it in the face, then run away. Whenever your pact timer is up, desummon, resummon, and Diaga as it runs over (in case it has shadows), Pred Claws again, then keep running. Repeat forever. You might take the occasional -aga IV but you should be able to survive it and Mounted Champion atma will tick you back up.