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pemerton I think I have a pretty good grip on the PCs in your group and I think one of your primary barriers is that each character is very "force-multiplication" driven. The only "dedicated" striker you have is not a swordsman but rather an immediate action driven ranged, Ranger/Cleric hybrid. Your Sorceror, from recollection, is an AoE heavy, buffed Blazing Starfall build. Then you have your Invoker who is right out.
Out of the two melee characters, the Paladin and the Fighter, one is built to be a very stout, single target Defender with secondary Leadership and the Fighter is built as an AoE damage/control machine; neither of which are particularly suited for the dynamics of a duel. With that being said, I think your Fighter has some built-in advantages that assist in creating a compelling duel; the forced movement involved in the build, above average AC and HPs. The first is really key. I know he uses a polearm so he probably has the feat tools for that that grants OA and buffs FM.
If I can make a few suggestions specifically to challenge that Fighter, it would be to:
1) Have an enemy that has a polearm as well and with it an aura 2 automatic damage to any enemy that is in the aura at the start of the BBEG's turn. That will incentivize and compel the Fighter to make tactical, movement-based decisions to move in and out of the aura, risking an OA on a move rather than taking the auto-damage. Further, the adversary should have forced movement rider (s) and possible an encounter power move action.
2) With the Fighter's and adversary's forced movement powers, Hindering Terrain and several Terrain Encounter Powers that are No Action riders triggered when a target enters the space would work great. However, I'd make these overt (as in power cards or annotations on the grid that get taken off when used). Eg;
Tangling Thicket of Thorns
Encounter - No Action
Trigger: Enemy enters the TToT.
Attack: Level + 3 (or 5) vs Reflex
Effect: Target takes 5 OG and is slowed (save ends). The TToT becomes difficult terrain until the end of the encounter.
3) I've been using end of round encounter cards for various fights since AD&D 2e Combat and Tactics. For duels especially, it will introduce round by round dynamism. An encounter card for a mobile hazard or random entropy is good; eg a log that rolls down a waterfall and attacks the duelists in the ankle-deep water, causing small damage, prone and sliding them to the edge of the secondary water fall (or just a slide in any direction of their choosing if it misses) livens things up quite a bit. The same applies for a stray arrow attack from a neighboring skirmish, or unseen slippery rocks. It animates the fight further by mandating mobility and brings all manner of genre "stuff" into play that would otherwise be outside of the scope of the duel.
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S'mon I'll look at those PCs and look over the available ED's tonight S'mon. The PC with primordial as atagonist is rife with material.