I always love running around Von5 on normal before an eV6 dropping grease on the up ramps to slow down the party members so I can get in front and get all the kills.
Or on the stairs in any number of places in the shroud to slow teammates down.
Point is, if the quest isn't particularly challenging then it isn't a big deal to spice things up with some minor griefing, as long as it is meant and received in good spirits.
Does SF/GSF:Conjuration, PL:Cleric, spell focus item: conjuration and PL: arcane disciple affect grease DC? If all of those did you could at least get the DC up to 21.