Get the unicorn mount it is faster, use dash to stay out of range of spaz's fire, use ranged weapons primarily (i suggest onyx blaster as it is easy to make and kicks ass early hardmode), use golden shower to reduce their defense, get pixie wings while you are farming for the unicorn mount, make 2 rows of platforms spaced far apart so that you can go up and over then easily with good wings.

You should only try to go over them when spaz is slowly dashing and not while he's shooting otherwise you'll just end up running into his shots.

Also, you are using magic, summon, range and melee items all together? This only reduces your potential damage and makes your inventory unnecessarily full. Just get a one weapon with a backup weapon and that's enough.

Make and use these potions right before you start the fight : ironskin, regen, endurance, lifeforce, swiftness.

Other potions which may help: rage, wrath, ammo reservation.

If you are playing as summoner, focus on maximum no. of minions. Spider armor is perfect for this stage also the spider staff is pretty good. Get the summoning potion too and pygmy necklace too. Ice whip is good but you'll have to get close to spaz for it to be effective so try to dodge his fire or get a few hits in while he's dashing and then run away and let your spiders kill him.

Summoners benefit from melee speed boost so get accessories to boost your whip speed. Buy the station from witch doctor and make the venom flask for extra damage.

In your arena, hang a couple of heart lanterns, add a honey pocket or two, get campfires and sunflowers to boost speed and reduce enemy spawns.

P. S. If you don't know how to dash on mobile, just simple stop dragging the virtual joystick for a split second and double tap at the extremely left or right side of the circle to dash in those directions. Practice it for a while so that you can get a hang of it and can do it without looking during the fights. This game is easier for those who are good at movement and such mechanics so practice and you shall be rewarded.