Tempest is one of the hardest tourneys but the possible rewards for completing hard tempest are sepix, sky stone or voltaic stone.
Easy (3 rounds)
Rewards: 10-15 coins, 0-1 random card
Initial hp: 80
Medium (4 rounds, last round ai has 3 ap)
Fee: 80 coins
Rewards: 5-15 coins, 2-3 random cards, 1 mystery or special egg
Initial hp: 100
Hard (4 rounds, 3rd and 4th ai has 3 ap)
Fee 100 coins
Rewards: 1-15 coins, 3 random cards, 0-1 exclusive fragment (sepix, sky stone or voltaic stone)
Initial hp: 120
1: Elec primary (with plant and dragon); your Wind is weak.
2: Wind primary; you can win this one in just a couple of rounds
3: Dragon; win as quick as possible before they get big
4: Dragon, Pinschi (not sure what else, I won too quick)
The main strategy of this tourney is dealing damage to the opponent as quickly as possible, so you don't lose much hp. Use monsters abilities to clear lanes, ignore low attack monsters. Following is a more detailed description of the individual rounds on hard level difficulty and the strategy for each round. (Note: the player giving advice for this tournament on hard (me) currently has a record of 37 wins and 39 losses. If you're reading this and have a significantly higher win percentage over a decent sample size feel free to edit as you see fit.)
First for reference below is the deck that the player is given for the tournament:
1 otti, 1 cubix, 1 zephyr, 1 garadion, 1 zug, 1 mooix, 1 stuky, 2 mamvolt, 2 wary, 1 volmice, 2 chargio, 1 ptery, 1 plumy, 2 rain stone, 1 lightning stone, 1 air stone
ROUND 1: Cards the computer uses (incomplete) = shen, drah, giraplant, zat, pigly, cubix, moox, chargio(?)
Strategy in brief = Play two creatures and kill computer creatures until you win.
First note that all the rounds of Tempest are more luck-based than other tournaments. Keeping this in mind for round 1 on the first turn you should almost always play two creatures, ideally a high health wind creature (otti, garadion) and an electric creature. Creatures to avoid playing on the first turn are mamvolt (4/10 and ability is wasted), zephyr (7/9, ability wasted), and stuky (5/10 on defense and computer likes to counter w/ an electric creature). After the first turn it depends on what the ai played, if you're lucky you still have a creature on the field to evolve and can clear any remaining computer creatures with one of your own so you end up with two lanes filled, one with an evolved creature (or you can play two more creatures to fill all lanes). If you're unlucky the ai cleared both of your creatures and still has one (or two) mostly uninjured creatures on the field. Typically the end result is either your creatures are cleared and the computer has one injured creature which you then finish off, or both you and the computer have injured creatures on the field. Often what happens is you and the computer swap creatures until the ai makes a poor move (like playing zat) then you gain the upper hand.
ROUND 2: Cards the computer uses (incomplete) = owly, malf, anchie, chargio, evolution stone, mamvolt,
Strategy in brief = Play high-health creatures, ignore anchie/mamvolt.
For round two for the first turn the best move is to play two high-health creatures or an electric creature and evolve it. The computer uses lots of creature damagers so you want a creature(s) that can survive the initial round or at least take out a computer creature. The reason to not evolve a wind creature is because the computer sometimes plays mamvolt and if the computer plays it you will likely be facing two uninjured creatures next turn and will take a lot of damage. Also do not play zug on the first turn as it is too fragile even after one evolution. After the first turn evolve creatures if able. If the computer has something like a mamvolt or barely injured anchie ignore it since they don't do a lot of damage and take too much effort to finish off, plus their abilities can't be used if they are on the field. The computer evolving these creatures is a good for you since that is an action that isn't spent killing your creatures. Only use electric creatures to finish off more threatening computer creatures such as owly or very injured creatures.
ROUND 3: Cards the ai uses (incomplete) = jastar, minigon, drah, drakeg, shen, wind/dragon stone, mantio, draku, rokuon
Strategy in brief = Evolve and pray.
Round 3 is the most luck-based of the rounds. Here the computer uses dragons that can get beefy very quickly (draku!) or have nasty abilities (drah especially). Since the computer gets three actions/turn and uses creatures that evolve into stuff you can't kill easily with your electric creatures' abilities the only answer is big creatures of your own. For the first turn it is usually best to play a creature and evolve it, ideally otti or stuky since they can't be insta-killed by drah though cubix or wary are also good choices (cubix is just good, wary evolves nicely and can move which is useful). After the first turn it depends on what the computer played, hopefully your creature is still alive and you can evolve it into its final form or play another creature and evolve it. Main things to watch out for are if draku was played, you need to kill it if able (if unevolved this isn't too hard, if evolved you're going to need mamvolt, if you can't kill it and it's evolved you're probably going to lose). Also if the computer has a relatively harmless creature on the field such as drah or drakeg it is usually best to let the computer evolve it so you have time to evolve your own creatures.
ROUND 4: Cards the ai uses (incomplete) = minigon, evolon, chiro, pip, pinschi, chipi, rire
Strategy in brief = Play lots of creatures and pray.
For round 4 the computer loves to chuck out pinschi which is why it is necessary to play two creatures (ideally high-health creatures), so one of them can take the hit rather than you. Again this is a very luck-based round, if the computer plays creatures like pip or chiro you stand a good chance, if it plays evolon and evolves it you're in trouble. Main thing is to try to keep all lanes covered after the first turn or two so pinschi doesn't get through and if there is a threatening creature like pinschi (first round or two the computer may just spam creatures so you couldn't cover all lanes and there's a living unevolved pinschi) or evolon kill it with extreme prejudice.