Any buff that increases your dmg modifier would increase the dmg your slays yield. These spells and any from their lines that give a damage bonus do so. Also there are some NPC's that have damage modifiers based on their HP levels, the weapon type used on them, or script mechanics. Fully raid buffed with Champion, mammoths strength rk 3, intensity of the resolute, and max attack modifer I was hitting for over 36k slays In FC1 raid instance. The named we were raiding has a unique trait that allows positive melee mitigation at certain points. Meaning you will see text slays for X dmg but the slash damage will be higher buy a certain percentage. In this case 45% higher.
[Sat Feb 03 17:58:01 2007] Gilthanos's holy blade cleanses his target!(25336)
[Sat Feb 03 17:58:01 2007] You slash Lorekeeper Bentolf for 36737 points of damage.
I would give you the stats for each of these buffs listed below but you can look them up for yourself on lucy.allakhazam.com
Intensity of the Resolute
Champion
Glyph of Destruction
Mammoths Strength
edit: This was done with Epic 2.5 and 6dmg aug (112dmg magic dmg +1 40 delay 58 dmg bonus)
Gilthanos <Triality>