Oh, here's another way to make fights in general last longer -
Encourage the creation of characters that are tactically sub-optimal.
The fact that fights will last longer if the PCs aren't as good at is goes without saying. It goes so much without saying that it needs to be said If the party has spent skills and feats on non-combat things, if they've chosen multiclassing for role-play rather than tactical reasons, your fights will usually last longer.
I found that darkness (from tieflings), coupled with making PCs make balance checks as they move into battle in said darkness helped make the battle longer. 10min/lvl is a long enough duration for any battle.