I'm not sure how your tuning goes but Valk casts her shield while she has def up from Sepulcher, right? I used to run a 4:3 Sepulcher with Valk too and I also got wiped around round 35+ so I had to compensate by increasing my damage per turn to get UNM in 2 keys. To start maybe try squeezing more Atk and Crit Damage to your Draco, and try getting your Valk to at least 7k+ Def. 

Ye, she casts shield while under def up. 

I could easily get her to 7k or maybe more, since now she has crit dmg gloves :D I stopped pushing her def on 6.4k since I thought it will be sufficient. Gonna try her out in full def mode.

I will try something with Jareg also. But I doubt him for some reason :O

If I remember correctly you lose Sepulcher's Def up on 2nd AoE, right? Maybe you can tune Jareg to fill that vulnerable spot. Also yeah, definitely push for more Def on Valkyrie if you can. Keep us posted on the progress!

A  couple  of  options  for  you  here....

Get  everyone  into  Stalwart.  Lifesteal  is  pointless  once  you  have  a  Valk  as  you  are  relying  on  her  shields  to  keep  your  team  alive.  Once  the  sheild  fails  its  a  one  shot  so  no  need  for  lifesteal.

seondly,  for  the  same  reasons,  if  yo  cant  survive  more  turns  look  to  do  more  damage  in  the  turns  you  have.  Defense  is  more  or  less  pointless  on  everyone  but  Valk,  so  get  them  all  into  Crit  Damage  gauntlets  and  push  up  your  damage  in  the  turns  you  can  survive.  Valks  sheild  is  whats  keeping  your  team  going  so  no  need  for  big  defense  numbers  on  your  team.  Go  damage,  and  your  numbers  will  come  up.