It also depends on the rules and the competitiveness of you opponents. The first thing I need to know is if the tournament uses official banlist or a French one or a different one (that also affects the life total). Because the deckbuilding depends on that quite a lot.
If you need a deck for both multiplayer and 1-on-1 I would most probably choose Nekusar because that kills the whole table at once and is fast enough even for 1-on-1 and you can play Time Twister in it.
You could possibly also play Marath trying to win via Twin/Kiki-Jiki or something. In 1-on-1 it is pretty good and it is the bane of blue decks in general.
Using the official rules you can also try Brago Staxx. This does deck does not really work well without all the fast mana, so if the rules be different don't play that.
@stsung apologies for the late reply. Working a double shift on three hours of sleep.
For the multiplayer they will be using official banlist and for duel they will use French rules.
Nekusar seams really nice. I have already been looking for cards that have either or both "each player draws a card" / "each opponent loses life" type effects.
Maybe even a couple "donate" effects.
As far as the players attending, I have attended three stores in my city within the last week, just to get a feel of what each of their league is about. One group is very casual and dislike competitive builds but is quite a large group. The other two groups tend to be more competitive, which I think are the groups I will most likely face at the tournament May 7th. They will all begin a new league next week. That gives me very little time.
I may just put together the Nekusar deck. They will not allow sideboards in either variant and will also be using the new mulligan rule (scry1) in duel commander.
Will post decklist later tonight after work.
So can you actually play two different decks or you are forced to play one that works for both? (because the banlists are different)
Nekusar is not good in dedicated French EDH. That format is pretty cutthroat. I mean for a Vintage player that does not need to be a surprise but you can just die on t3. Happens and many decks are trying to kill fast even though more people play control with combo finish so they are slower.
In multiplayer if you want to win you can go all-in more or less but in French you'll be better positioned if you will use a control shell unless the tournament is going to be really big. For big tournaments (50+) I used to play all-in combos actually.
You should be looking for Time Twister effects (there's like 12+ of them), card draw and then some cards that helps kill your opponent (for example Fate Unraveler..or how it is called), underworld dreams ...you can even abuse megrim effects if you won't take storm path