Thank you ! It worked really well when I ran it. They had the whole "recruitment experience", then plenty of RP interaction with the mercenaries, ship's crew and locals as they sailed down to the exotic port near the fighting. Then I had them go off on their own on a couple of recce missions before the whole army tabbed across the desert to fight the big battle.
Lots of exhaustion and heat stroke checks later, they arrived at the choke point of Wadi Naffatiyeh. There, they were in reserve as the battle started, but I kept them in the narrative by having undead manticores fly overhead firing tail spikes and a few airborne attacks from (frankly terrifying) undead sphinxes.
After enough of that to give them a flavour of the desperate battle being fought, but before it got boring, I sent them off on their special mission - to go under the battlefield with a scroll of Earthquake and lots of ways of setting off explosions. They had to fight through undead counter-miners to find the underground naptha pits and blow the whole shebang before the army above ground was overwhelmed.
Having a serious time constraint made things thoroughly exciting, and stopped the "I've just used a couple of spells, time for a long rest" syndrome. Everyone had a lot of fun !
Yes it's interesting to be set with a fix amount of resources and/or time and see what you and the other players come up with to achieve their goal!