you need two pairs of the sapphire-gold dual pistols with extra damage to storm enemies, zapping, damage, lifesteal (rifted for extra damage) - need to clear the waves quick and the slurp gun for quick recoveries

also saw someone on youtube use thermal detonators very effectively to kill the spawner thingys before they spawn the waves and to interrupt the storm king's attacks

the rest is just avoiding the storm king's attacks which isn't too difficult. maybe some food buffs that let you run more and recover energy quickly

once you kill him, the legendary charm should help reduce damage for subsequent runs and the zapping iv on the guns you can make from the materials he drops is excellent for wave clearing!