Fire Slinger - You're *way* too powerful. Being untargetable and a 0/6 makes you almost indestructible. Paying less mana than an Arc Mage and NOT discarding a card for the same damage makes you even more broken. If you're going to be balanced, you're either going to have to do 1 damage to target creature or player, or be a 0/3. Your choice.
Gatha, Tolarian Renegade - Reduce the activation cost by 1 and make it a +1/+1 counter. See also Dragon's Blood.
Jhoira, Tolarian Artificer - Remove the ability to search for artifacts in your library. Otherwise, remove the card from the set. Currently a reuseable and uncounterable Tinker that doesn't require a sacrifice.
Davvol's Flying Disc - Increase the casting cost to 4. Remove the ability to keep it tapped, and add an activation cost of 2. (Flying and first strike are a truly insane combination.)
Sea Dragon - Remove the flying ability. (It's unblockable; who cares if it flies? Also, why would a SEA dragon fly?)
Marsh Dragon - Change the special ability to either 'BB: Target creature gets -2/-0 until end of turn' or 'BB: Target white or green creature gets -1/-1 until end of turn'. (-2/-0 because Marsh Gas comes to mind, and B for -1/-1 to any creature is WAY strong.)
Treetop Dragon - Change the activation cost to GG.
Hill Dragon - Add 'If at any time you control no mountains, sacrifice Hill Dragon'. (Where will a Hill Dragon live if there are no hills?)
Badlands Troll, Savannah Troll, Tundra Troll, Taiga Troll, Delta Troll - No changes.
Squirrel Den - Add 'Sacrifice two lands' to the activation cost. (Sure, I'll make three 1/1 creatures EVERY TURN, more if I have more mana...)
Walnut Tree Grove - No changes.
Desperation - No changes.
Seagull - No changes. Mmmmm, Flying Men...
Master Wizard - Change to 1/1.
Sea Snake - Change casting cost to 3G. (2/1 is 1G, +1 for uncounterability, +1 for protection)
Recharge - Change casting cost to 2UU, or change to 'Gain 1 life'.
Confuse - No changes.
Book of Fire, Book of Life, Book of Death, Book of Rage, Book of Knowledge - Either remove 'When ~this~ is placed into your graveyard, you may pay 2 to shuffle it back into your library', or increase casting costs to 3.
Crushed Rebellion - Change casting cost to BBRR. (Remember the headaches Kobolds caused when people said that because they didn't have a casting cost, they were colorless creatures?)
Cheetah, Jaguar - No changes.
Flickering Powerstone - Change 'Cannot be the target of spells or abilities that cause Flickering Powerstone to untap' to 'Flickering Powerstone can't be the target of spells cast by its controller'.
Nature's Prayer - Change it into an artifact. Have it tap and sacrifice to produce your choice of abilities.
Psychic Backlash - Change its casting cost to 1UUUU. (Since when does red counter spells in general?)
Plane Shift - Cut casting cost to 2UU.
Titania's Reward - Change to '2GG, Sorcery, all creatures you control get +2/+2 and trample until end of turn' to keep it from being a near-exact copy of Overrun.
Image Focuser - Cut casting cost to 4. Change 'When Image Focuser comes into play, pay half your life, or sacrifice Image Focuser' to 'Cumulative Upkeep: 1 life'.
Concentration - Cut casting cost to 1UU. Reword to 'Until end of turn, you may choose to pay 2 life at any time. When you do so, draw a card. You may not pay more than 4 life in this way.'
Silence of the Lambs - Cut casting cost to 2BB. Rename to 'Outnumber' to avoid lawsuit.
Ransom - Change to:
Ransom XUW
Enchantment
When Ransom comes into play, choose a creature. Put X counters on Ransom and remove that creature from the game.
At any time, the creature's controller may pay X. If he or she does so, sacrifice Ransom and return the creature to play under its owner's control, tapped.
Field Hospital - Change 'gain 5 life' to 'gain 3 life'.
The Dark Side of the Moon - Change 'All non-black creatures get -2/-1' to 'All non-black creatures get -1/-1' or 'All non-black creatures get -2/-0'. Cut casting cost to 1BBB. Change name to 'Moonshade' to avoid lawsuit.
The Matrix - Cut casting cost by 1. Change 'sacrifice all creatures' to 'sacrifice all creatures you control' to remove requisite stupid questions.