Fire's strength lies in its powerful offense. However, it sacrifices all defensive ability for this. Fire is Offensive.
Well, Fire has the strongest stall in the game, so "sacrifices all defensive ability" is a little harsh. Even I'm not that rude to Fire!
Anyways, srm, what's most important to your event is ranking between monos; since many of the decks used will be monos and a strong mono is necessary to ever get anywhere. Here's what I believe is an accurate
ranking in mono vs mono games. 
Entropy finishes first primarily because of Discord, but also because of strong support and the ability for both rush and stall. Discord rush alone can beat any mono that some elements could possibly bring. Entropy also brings Dissipation and Antimatter stalls to anyone that tries to defend against Discord. Entropy can run multiple different powerful decks that dominate mono environments. In the event, Entropy could be handicapped by not being allowed to use weapons.
Earth comes in second because of its speed, denial, and control all packed into a single versatile deck. Earthquakes tear through mono decks because most Elements are forced to use only two quanta sources, Pillars and Pendulums. Since Immophoenix's nerf, Graboid is the strongest creature in the game and can easily fit into a mono deck if a Time mark is used. Basilisk Blood is one of the best CC cards in the game because hardly anything can resist it. Against an opponent with heavy control, Earth can use the resilient Stone Dragon with 10 hp. Against an opponent that relies on permanents for defense, Earth has Pulverizer. Few monos can get in the way of Mono Earth. A potential handicap would be forcing players to use a mark of a shield piece they have, which prevents Mono Earth with a Time mark for Graboids.
Mono Fire is definitely capable of beating a lot of decks. Fire Bolt can kill most creatures after a few turns, Rage Potion provides good creature control, and Fahrenheit works great with the large amount of Fire quanta in a Mono Fire. Phoenix is strong against opponents with damage based CC, and Lava Golems work great against opponents without damage based CC. Add in a great PC card, and Fire has almost everything it needs. I'm not sure what the best way to handicap Fire would be, maybe limiting spells.
Two cards make up the core of a standard unupped Mono Aether, Dimshield and Phase Dragon. The combination of a shield that blocks damage sources from entire Elements and a Dragon that is nearly impossible to kill is too powerful for many Elements to handle. Aether could be weakened by limiting the amount of Shields that can be used to, say, 4.
Death has a great mono rush with efficient creatures and Poison to get past shields. Bone Wall is a great shield that can delay the opponent for 1-2 turns, which is often enough to win the game. But Death only made it as far as fifth because it lacks PC, Denial, or any fast way of CC.
Mono Darkness has versatility with CC, PC, Denial, Healing, and strong permanents. It works well as a mono with turn-2 devourers and Drain Life, but it has two main weaknesses. Mono Darkness can't make a fast rush since it really lacks a creature with good attack for its cost. Mono Darkness also struggles to find strong CC, since Drain Life requires much more quanta than most CC cards.
Water has powerful CC because of the Squid/Toadfish combo with Ice Bolts or Freezes for support. Toadfish are an efficient attacking creature that become even stronger with an Air mark for their ability. However, Water lacks PC and strong permanents; its shield and weapon are weak.
Air can block entire decks with Wings, Shockwave, and Owl's Eye as well as breaking stalls with Unstable Gas. However, Air lacks PC and a fast rush. Its defense can be strong if unexpected, but any Mono can use Dragons to get past Wings when playing Mono Air.
Light has the strongest healing in the game supported by a great shield and a resilient Dragon and Weapon. However, what hurts Light is its lack of CC, PC, and a fast rush. Light is more of a duo Element.
Although time has good cards, it isn't an Element built for mono vs mono battles. Reverse Time is great against rainbows and duos with stat-raising cards, but it isn't nearly as strong against monos. Ghost of the Past works best only in duos that can use cards like Nightmare and Fractal.
Life is like the Element of almost being worth using, except not. Its has no PC, very weak CC, a subpar weapon, and the two worst shields in the game. Its creatures, which are supposed to be its specialty, are around average. Frogs are cost efficient but easily killed, and Cockatrices have nothing special. Mitosis is a worse Fractal, and Heal is a worse Sanctuary without the protection against denial. Life's only quality card is its dragon, but with no other strong cards for support, not even its best card is worth using. Without a duo element to rely on, I don't see how Life could make it far in this event.
Gravity ends the list because it simply cannot function as an unupgraded mono. Its cards are all either underpowered or working best in a duo. While it has some valuable cards like Black Hole, Acceleration, and Momentum, these cards require help from cards like Discord, Voodoo Doll, and Deathstalker, respectively. Gravity is a weak Element that is crippled in a mono environment.
