I thought I'd check what the actual damage output from the Death card proves to be - I'm not sure about the leader ability as the loss of hp of any team will impact survivability.
I therefore set up a mono-blue team with Death in just to check the impact of its activated ability to see if I may think about using them in GW as opposed to panzers. I recorded the 'extra' number of lines of damage over almost 200 activations of the ability at which point the averages started to stabilise.
Results
The average (mean) extra output was just over 8 extra lines vs. single target - which assuming around 6000ap (after modification) gave damage of 49100 per activation. This is clearly better than Water Panzer for single target (with zero defence)
Against multiple targets there is a drop off compared with Panzers as often no '5 line' is made but the additional 32300 damage (based on 6000 ap) per extra target still is slightly better than water panzer.
Around 1 in 8 activations though produce 0 damage, and around half produce equal or less damage than a panzer as the average is boosted by the 11% of results that produced 20+ lines (always with 5 line) so there is no consistency of output.
(Table attached at foot of post)
So, would I use it in Guild War over panzer? Damage is higher, but since the average is skewed by those activations that throw out say 120000+ raw damage then there is a chance of overkill.
Defence reduces the damage by up to 6 times that over panzer as it will affect each of 6 sources - high defence (3000+) stymies the damage.
In Guild War the activation of the damage takes 2 turns so you need to be sure you don't die before second half of effect and trying to synchronise the effect with Grim Reaper may be tricky, especially in this next Guild War if it pans out the same.
The final consideration for the Guild War is that the rewards are biased towards consistency of damage - two 50% damage results give more than a 0% and a 100% and this doesn't favour the Horsemen cards.
For players without Panzers Death could make the Doll/Healer mono deck fun in Guild War as you can get 2 or 3 activations so with Medusae in the team there is a small chance of boss kill once the right element team can be made.
The Horsemen are going to be fun to use and although not as reliable as Panzers I can see them getting some use
(sorry about format of table)
Lines Frequency
0 23
1-3 39
4-6 30
7-9 17
10-12 22
13-15 12
16-18 5
19-21 6
22-24 2
25-27 2
28-30 4
31-33 2
34-36 0
37-39 0
40-42 0
43-45 2
46-48 1