Well, a Monk in the mid-row is definitely not as safe as one in the back row, but from what I've seen in the Indonesian Titans, the midrow Monk isn't getting sniped or killed very quickly. I think that with the addition of Pegasus gears, the patch that reduced everyone's damage, and the job damage reduction that magic mercs get vs. ranged mercs, that it's entirely possible to put a Monk in the middle. As far as magic is concerned, he'll remain safe as long as you always have a source of dispel, Empress being the more efficient choice because she HGs as well as snipes. I'm not an expert, just stating what I myself have observed.
Personally, I would re-arrange your form so that when you do go to HG you don't get your main in it too. (We all know how that works out for staff(M)) Keep in mind that this is entirely optional, so feel free not to use it if you don't mind HGing your main. Just be careful of people who might try to snipe you. Also, be prepared for when Mwindo gets her purification skill. It reaches to the second row, so people can still set you up if you HG too early.
Also, if you wanted to do Beast Soul AND Seth's Will, you'd need to put an Oracle in there. Unless you meant Brutal Will on your main. If you did though, you'd need more mercs that hit multiple mercs in one hit than the ones you have now. The Puppeteer and Sheriff are fine, but the Vampire only hits two in a column. Also, with Rin and Christine being spiking mercs, the Vampire becomes less effective being a sniping merc. moreso than a spiking one. So, Hell Flame with a support Staff(M)? Viable. The BS/Seth concept? Not viable with your current mercs. The best that you could probably do regarding BS AND Seth together is this. This way, you have a better chance of getting all of the mercs on the frontline, and all of them spike. However, this isn't viable because you'd need to take Blazing Earth on your main to aid the spiking, use your main as a healer since there's no more Vampire, AND still be able to get Hex of Darkness out every three turns.
My recommendation is to pick one of your two forms of offense to make your primary, and use your secondary offense to help it accomplish it's goal. For example, in my signature is my build. My main form of offense is Hell Flame, but I also have a little bit of stun just so that more than one activation is required to completely dispel it unless they have a Minstrel present. I can also use my Sorceress to seal the Exorcist's Speed Cast or Valkyrie's Awakening so that the Monk has very little AP
after taking care of the Hell Flame. The reason I'm saying this is because you seem to be holding Hell Flame and your left-clicking on the same pedastal, which means that you may have problems with the two clashing with each other in an actual battle. If not, then you're playing very well. But if you are, then you may need to re-work your strategy and determine which is your primary/which is your secondary. I wrote a lot and I hope it all helps!