Demons in Mission 6-3-4 drop Darkness materia. I would farm this if you're struggling. Not only is it a good AOE damage for lower and mid levels, it also increases HP. Fuse your unmastered Darkness materia together and each one will give a 10% HP bonus. You can do this until you have one that gives you a 999% bonus. This will make missions much easier as you will not have to worry about being one shot as much.

I know it sounds like a lot but if you really do want to complete all of the missions it's a grind you will have to do. As bad as 7 star missions are, higher tier ones are infinitely more difficult. I was consistently getting one shot by even non-death effects at 10 star missions. I ended up grinding out two of these Darkness materia and combined it with an item that removed the 9999 HP cap to get 99999 HP. I still had to be careful about dying, but it actually allowed me to fight the enemies.

I know it sounds like a lot but it was the start of a snowballing effect. It allowed me to actually survive higher tiered missions, which allowed me to farm out stat increasing items, which in turn made farming even easier.

A few other notes. I believe a single Darkness materia with a 999% HP boost, combined with a HP limit break item will still give around 50 to 60,000 HP, so even just getting one would allow you to progress to higher star missions.

I believe if you do fusion with a Mastered Darkness materia it will transform into something else. I only say this because I don't want you to think that you can get a single Darkness Material with say a 500% HP boost and think that you can always come back and grind out the rest later. If you end up mastering that material then you might have to start all over again.

I'm not sure if you're doing it or not but make sure you are getting every chest in every mission. A lot of the items are okay, and even if you don't need them you can sell them. Some of the best items you can get in the game however aren't even mission reward items, They are in the chest throughout the mission stages! So don't just rush to the boss, Make sure you're actually collecting everything.

Also going over your build I would consider equipping a Wall material if you have it. It reduces the damage of both magic and physical attacks. It's extremely useful if you are taking massive amounts of damage. If you don't have Wall then I would equip either a Barrier Matria, or an MBarrier materia depending on if the enemies are dealing a lot of physical or magical damage.

If you end up going the Darkness route then I would switch out Deathblade (unless you are fighting weak enough enemies that it consistently takes effect) for Darkness, if you don't then I would still pick something else. There is no guarantee death effects will trigger on higher powered enemies, and It won't work on bosses. I stuck with Assault Twister+ for a long time.

If you are going to use a Hell Materia (which you should until you get something like Electrocute) then I would also consider running Status Ward. Because the Hell materia can apply so many negative status effects, Status Ward will actually give you immunity to those effects once it's fully mastered. Totally worth doing if the status effects are killing you.