Can you craft poison pots? Might be worth a try.
If you have enough strength/dex perhaps buy a longbow and a ton of arrows and keep firing them in from the fringes. If you have your bow selected when you summon your horse you can fire from horseback and remain mobile.
How many summons are you getting/making? I get 6 right at the start and a further 5 at the top of the hill.
Then I was just running around the battlefield using the dunes as cover and re summoning whenever I saw a glyph re-appear. The glyphs tend to move around the battlefield a bit though so keep circling. I must have summoned most of the characters about 4-5 times before the summon spots seemed to tail off. Maybe they just moved somewhere else further up the battlefield, though, I’m not sure.
With the dragon rot incantation, are you holding r1 whilst locked on to make sure you do a fully charged cast? Takes a good few seconds to fully cast.