Helix m.acc/m.atk x5
Helix m.acc/m.atk is the obvious 5/5 choice; Arts Recast the obvious 0/5 choice. It took me a long time to decide between meriting Modus Veritas and Sublimation; really they're both small upgrades. If MV merits reduced the recast I would've gone 5/5 on that for sure, but +25% damage on a helix every 10 minutes, while nice and all, just wouldn't be used nearly enough to justify the merits. If I were a 100% Dark Arts at all times SCH, yeah, MV all the way, but that's just not the case for me. Not that Sublimation merits are awesome or anything, the increase to your efficiency is small, but it's a bonus that you'll receive pretty much constantly no matter what situation you find yourself in. Also, they're nice to have when you want to go into an event with the largest possible Sublimation stock, like Limbus. Throw on some HP equipment to get to 1400HP, and that's a 400MP stock right there. Again, I don't want to make it sound like Sublimation merits > MV merits for everyone, that's just the choice I ended up making, and I'm pretty happy with it.
Stormsurge and Helix m.acc/m.atk are no-brainers of course, but the remaining category 2 merits are difficult. I agree that unlocking Enlightenment is something everyone should do, simply because of the versatility if offers. However the last 4 points are kind of difficult to decide on. Both Light Arts Strategems are useless of course. Focalization seems the obvious choice and that's what I ended up meriting, but I did give serious thought to Equanimity.
My logic goes like this: I'd only really ever use either during Tabula Rasa nuke spam, during which I would have the appropriate Storm + Klimaform up, making Focalization kind of redundant. On the other hand, Equanimity is completely useless while soloing. However, in a party situation, Equanimity could come in handy by eliminating the need to hold back, allowing you to spam as many nukes as possible without fear of death. In the end I went with Focalization because my LS has me on Light Arts during events more often than not, but I think Equanimity deserves a serious look, depending on what you use your SCH for.
Look at it this way; Category 2 Strategems are essentially Tabula Rasa upgrades, so look at the situations in which you find yourself using your 2hr ability most, and merit accordingly. Don't bother looking at the Light Arts strategems; Capped -Enmity is attainable through gear alone, and how many White Magic spells do you cast that accuracy matters enough to use two charges on, or cast during Tabula Rasa?
As for other merits, Enfeebling > Elemental > MP <> INT, depending on your needs. If you really have nowhere else to put your Combat merits, Evasion never hurts. Under Other, one can make a case for Enemy Critical -%, but I went with Enmity -% and Spell Interrupt -%.