I understand the system you're using is RMSS/RMFRP but you posted in the general RM thread so here's some info from RM2 that may help you:
Elemental Companion has some fantastic Greater Drakes with three different age ranges. The ones that stick out that would be great to send against your mage are these:
Chaos, Young lvl 25 -- Hits 350G (The 'G' is for calculating HP if you choose to send a Drake greater or lesser than level 25)
Chaos, Mature lvl 50 -- Hits 650G
Chaos, Old lvl 75 -- Hits 950G
Nether, Young lvl 25 -- Hits 300G
Nether, Mature lvl 50 -- Hits 600G
Nether, Old lvl 75 -- Hits 900G
Time, Young lvl 25 -- Hits 300G
Time, Mature lvl 50 -- Hits 600G
Time, Old lvl 75 -- Hits 900G
Those three types stand out for a few reasons. Primary the number of HP, secondary the level, tertiary the type of crit.
That's where it gets fun!
Your concern was a creature that can handle 1200+ HP in one attack. That's tough to deal with but this is a start. A flying Drake has a bonus to DB and if your mage is casting a ground based illusion, the Drake can take flight or just stay air born. Some Drake can teleport to other planes and some are invisible.
The next one is the level of the creature. In RMSS/RMFRP, do the targets get a RR vs the Illusion or realm of magic? I've never player RMSS/RMFRP. The youngest Drakes at level 25 are higher than your mage. If you go to level 50, it's considerably higher than your mage, I would assume that a level 75 Drake wouldn't even notice someone casted a level 20 spell at it.
The crits are where the fun happens, but you would certainly need EC and/or RMC-III for the Attack Tables and Crit Tables. Here are some samples:
RMC-III has the Physical Alteration Table
Chaos Cits inflicted: Acid and physical alteration (physical damage from acid and physical alteration. Turn that leg into a crab claw!)
Nether Crits: disruption and stress (Reality checks, orientation rolls at varying difficulty, physical alteration. Turn the arm into a mushroom!)
Time Crits: time (Time Crits reduce the targets stats at random and can also hyper age the target into dust)
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If RMSS/RMFRP allows RR vs Illusions or realms of magic, then even a young Drake is +5 levels over the mage. All of the Greater Drakes are extremely intelligent and have access to spell lists. There must be a spell list that protects the caster from Illusion spells. I haven't even tried to look through the spell lists, but I'm sure there's a Dispel Illusion or something that gives at least a bonus to the RR vs. Illusion.
For example, the Chaos Drake has mastery of Chaos Mastery, Chaotic Weapons, Chaotic Armour, Changeling, Metamorphose spell lists. (3x Chatoic Lord base lists, Warlock Base, Moon Mage Base) 3x level PP
I didn't list the stats, but the Nexus Drake (not a Nether Drake) is a master at teleporting with mastery of Gate Mastery and Spell Wall lists and could really make things hairy for the mage hiding in a tower, especially if there suddenly is a Drake in the same room and only an arrow slit through which to escape
Vacid Drake has mastery of Inner Wall list and Protection spell lists (Ranger base and Cleric Base)
Aether Drakes pop in and out the the aether plane at will at different locations in the physical realm. They have mastery of Ethereal Mastery and Invisible Ways.
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While you can't always send a Greater Drake at the party, hitting the mage with a decent Chaos Crit and a physical alteration just once can be very devastating. A suddenly old mage with stat degradation will weaken the spell casting ability. Aether crits can teleport the target to the aether plane. (I would assume would be instant death unless the target has Hostile Environments - Aether as a skill. ) These suggestions may seem extreme, but you're dealing with an incredibly OP mage, for game play balance, something equally nasty should be fighting her.
She is using the 9 'extra' options to give the spell 2 'feels', and 7 doubles on radius (so the radius is now 1280 feet).
This is the key component though. These 'options' are they optional spell casting rules that you allow in the game? Optional rules always run the risk of something OP occurring and a lot of the optional rules I've seen state Option 1 or Option 2 or Option 3, not "use all of the options listed here."
The best choice is to cull those options if they are optional spell casting rules. Allow only one, maybe two spell casting option. For game play balance, in our gaming group, whatever options the players use, the baddies get to use. When one of my mage players decided to try using Verbal, Somatic, Token, and Spell Holding to increase his spell attack at the boss... I made sure my NPC spell caster was able to do the exact same thing and the party didn't like the result.
Conversely... since the player used all four options for his spell, if he didn't use all four options, his spells suffered penalties because he wasn't completing the entire spell process. He was leaving out steps in the spell casting process so spell failure was increased as well. When his hands were tied, he couldn't cast his spell because he needed the somatic portion of the ritual.