Can't even beat the Clarisse Hero Battle on Extreme...

Try going to Burger King.

What I can confirm is, that a Horse emblem, especially with a Gronnraven Cecilia stomps the shit out of Infernal mode. Grats on the kill :D

I used an ike to bait the thief and green mage (xander works too)
Then with camus i put him 2 spaces away from ike to the right with reinhardt below camus
Then reinhardt kills clarisse then reposition reinhardt with camus and have ike hide in the corner for the rest of the battle.
The red horse kills himself from camus, then move camus to kill the thief and reinhardt to kill the mage thus just leaving the red sword unit to die from camus.
Hope that helps.
Oh just make sure camus has goad cavalry because this insures reinhardt will kill clarisse.

-- On Hard and Lunatic --
Take any bulky Red Distant Counter user (Black Knight, Xander, etc), a dancer, a bulky Green mage running G Tomebreaker (Julia is excellent), and one of the quest units.

- Turn 1 -
Have the DC unit advance into the line of fire and destroy the breakable wall (they should be one square outside of the Green mage's attack range). They need to be able to survive/eliminate the dagger user, Clarisse, and mounted sword user during the enemy phase.

- Turn 2 and onward -
Have the DC unit withdraw and have your bulky Green mage advance into firing range of the Green Mage (dance to get everyone in position) - they should destroy the remaining two units but it might take a few turns. Maneuver your Green Mage onto the defensive tiles as soon as possible.

-- Infernal --
The same basic tactics will work in Infernal but it is more challenging as there's additional sword user threatening our Green mage and the mounted sword user now has Armorslayer+ (only a problem if the Black Knight is your bulky Red Distant Counter baiter). You might have to swap out the quest unit for someone useful...but should still be able to pull it off.

I beat infernal the first time around and (M)Robin kills everyone except the green and blue mage.