To be honest, that took way more attempts than I thought it would. I can't count how many times I got halted by a high level undead blocking the path early on, and it was not uncommon for the blackspace behind said zombie to hold one or two of the glyphs on the level. The Firstborn was definitely the harder fight of the two bosses given such high damage, though clearing the curses during the descent to the Matron was also a pain as it required nearly completely clearing the map on my winning run to get rid of all the curses I accumulated. I'd place it around NL in terms of difficulty, as the map layouts of both can easily screw you over in the early game and you're kinda forced to fight pretty difficult bosses at lower than optimal levels.
Just got VS and VT VGT before I can finally put the Vampire down and move on to one of the other classes for a complete clear. Now gotta refill my gold before investing in the money drain they'll both be.
EDIT: First VT VGT attempt went pretty well up until Horatio. Never found platemail, so I was required to use bloodcows during his 120 dmg phase, which meant I only got him down to ~1400 health before I had no more resources left. I'm near certain that I could have beaten him with that extra 20 DR, as that would have knocked his damage down enough that I would have just barely been healing from his attacks. Also, having patches be more even in his boosts (I got SEVEN mana from him alone - 25 total with items) would have knocked down the resources required dramatically.
Really, the first 3-4 towers were the hardest, then as soon as I got the two sensation stones the build took off rapidly. In the later towers I was healing off not only the tower's meatmen but also the dragonspawn as well, which drastically eased the strain put on my bloodcows. I got lucky in not having a bloodless floor but a late grim malus doomed three of my bloodcows and might've been the final nail in the coffin for the H fight.
I had wanted to post that finding both or one of Getindare and Bysseps would make the difference but it seems you already done it, 