This is something I consider myself to be pretty good at. I value mechanically interesting fights pretty highly, so I've given this a lot of thought.
Anyway, here's what I do:
I look for gimmicks. I'd divide gimmicks into three broad categories: environmental, character, and narrative.
An environmental gimmick is a battlefield with unusual mechanical effects. For instance, one of my recent fights took place on an unrealistically-large tree in the Nevernever. If you wanted to fight close up, you'd need to make a couple of sprint actions to get up the tree. Plus there was a landmine at the top. A brief list of environmental gimmicks:
-Zone borders.
-Traps and landmine-wards.
-Environmental hazards.
-Very small battlefields.
-Very large battlefields.
-Zones with special traits.
-Unusual enemy formations.
-Unusual zone maps.
Then there are character gimmicks, which are unusual traits that make the combatants in a fight mechanically interesting. For example, one of my PCs in my PbP game launches massive Evocations but has no Toughness or Speed. If every combatant fights differently, things are more interesting. Basically, I try to diversify. Here's an incomplete list of what we have to pick from:
-Numbers.
-Unbalanced offensive/defensive capabilities.
-Enormous damage with low accuracy or vice versa.
-Incite Emotion-based combat.
-Evocation-based combat.
-Crafting-based combat.
-Custom Powers like Extra Appendages, Damage Shield, Dangerous Aura, etc.
-FP-based combat.
-Physical power-based combat.
-Stunt-based combat.
-Unusually long or short ranged combat skills.
-A mechanical weakness like a Catch.
-Summoning and other in-combat thaumaturgy, courtesy of Sponsored Magic.
-Poison.
-Grappling.
-A build that promotes excessive maneuvering/blocking.
Then there are narrative gimmicks, basically odd situations. Like an ambush or somebody being too crazy not to kill their own allies. Or like giving your PCs some friends/people to protect. One example would be the tree fight I mentioned earlier, where the two PCs had three golems to help them out. Unfortunately, I ended up having to run one of the PCs in that fight too, so the NPC ally-to-PC ratio got messed up. But that's beside the point. Some random narrative gimmicks:
-Everyone is fighting over a Macguffin.
-One side just wants to run away.
-Somebody turns traitor during the fight.
-One side has an unfair advantage, like an ambush.
-The teamwork on one or both sides is awful.
-The enemies of the PCs are fighting one another as well.
-Anything involving Compels.
-The PCs are fighting alongside NPCs.
-Someone is specifically targeting someone else.
I mix in a few gimmicks. Then I look at my players's capabilities carefully. I imagine what a typical attack from each fighter will do to each other fighter. If things seem wrong, I change things.
Making a fight interesting isn't just about presenting a challenge, though that's part of it.
PS: Accuracy 5 weapon 5 defence 5 is nothing special. Skilled mortals can reach that level, no problem. Not sure why you'd be having problems with that. Are you not giving your mortal goons armour?
