To fight a Summoner?
There he stands, in his shiny green armor, motioning at you with his book light. You grab your weapon and get ready to move in. The SMN starts charging a spell as you approach. Then another. Two more come out as you're closing the distance.
And then the TICS START.
Bio II. Oh god, you're losing your lunch. Your muscles are inflamed and your whole body feels like it has been poisoned. Its vomiting, diarrhoea, and nosebleed all wrapped up into one and coming out every ******* you've got.
Miasma. You're choking. You can't breathe. Your natural stamina is being sapped away and your vision is blurry. The whole world is spinning and your lungs are on fire. And you somehow know that healing magic is no longer able to save you....
Bio. You're pushing a fever of 103 almost instantly. You're already dehydrated and you have a migraine that could kill a bull elephant on top of all the other symptoms.
Thunder. You feel like you just got Tazed. Your muscles are convulsing wildly and your skin is blistering off from the heat of the current.
You're finally up on the Summoner, axe in hand, and you can already barely move. Your body is a wretched hive of agony from which the sweet release of death is barely minutes away. You struggle to lift your weapon as the whole planet whirls around you in a blinding haze of rotting flesh, diseased fluids, and medical depravity..... If you can just get in one swing! He has paper for armor, come on!!!
The Summoner is looking at you..... He's pointing his left index finger out at you like a gun. And he's grinning.
From the POV of the enemy, a SMN must be about the most terrifying job in FFXIV. The others all just kill you in various ways. Not rot you to death from the inside out in a thousand horrible ways. Even their "burst" damage move just accelerates the rotting for a second to eat chunks out of you. Jesus Christ.