hehe, first off what do you need it for? Do you plan to make a map for playing with friends on BNet? How many players, etc?
For e.g. if it's more than 2 players but only 2 will be in the duel then we may need to dabble in variables, but if it's only 2 players anyway, you could probably get by without them...
Variables - Briefly
The best way i've seen variables described is as 'filing cabinets' in which we can store information.
We can store almost anything in variables, units, unit's health/mana values or groups of units and much, much more.
This is essentially allows us to make the game 'remember' certain things, we can call upon later in triggers - which is why it is essentially like storing information in these 'filing cabinets', which we can tell the game to look through later in order to use stored information in a trigger.
Variables are split into several types, each type can hold certain information, imagine it's like colour coded cabinets, you
can't put a green folder in the red cabinet.
Some common variable types are: Real, Boolean, Integers and Units.
Real = Real Values, these are continuous numeric values, e.g. they can have decimal places, so things like unit health and mana/time of day can be stored in these.
Integers = Integer Values, these are 'whole' numbers, so only whole numbers like the amount of units in a particular area/region of the map can be stored in these variables.
Units = Exactly that, you can store units in these variables, so if we were to store a unit, then kill it, we could make the game bring back exactly the same unit, because it will remember/have record of exactly what it was like.
Boolean = Boolean variables store 'True' or 'False' values and can really - as far as I know - only store/be equal to two values... 'True or False'\'Yes or No'.
Unit and Boolean variables would probably be most important to your duel system, storing the combatants and checking whether one is dead/winner/in the arena etc...
Your duel system and things you could do, including and excluding variables
There is so many ways you could go about this... really your best bet is to find a map you have seen such a system on and assuming it's unprotected, open it and see how the triggers work in that, or check the tutorial section and see if anything helps - I don't see very much point in me handing you a trigger if you barely understand how it works, else you won't be able to tweak to your liking, however if your desperate/can't find anything post back and i'll see what I can do!