Yes, in your position, with the units you have available, the list needs an entire overhaul.
Rangers & Dark Reapers will be destroyed on such a small board. Their excessive range for their weapons is no use. That, and they'll be caught and mobbed. That's 215 points you're essentially giving him.
The Warp Spiders are tricky, and it depends purely on how good you are at using them. 2 units of 5 can be very good at distracting, hitting the rear of vehicles... and Str6 shooting does wound marines on a 2+... however, actual 'damage' is beyond them, due to small unit size. A round of shooting from bolt pistols will probably send them running. A larger unit of say, 8, would be more viable. You'll save a couple of models cost, and the concentrated fire will reap better results.
War Walkers are an option against him, but remember to run all of the same weapon. Starcannons would be a good bet, especially if he has Mephiston and Terminators wandering about - Outflank them and slam some Starcannon into them. Armour save 2+... and what?
Fire dragons are still, sadly, a suicide unit. And anything short of a Land Raider/250pt+ beastie really isn't worth their time. Yes, you can vaporise a couple of terminators... but that's all you'll accomplish. Some clever DA firing could accomplish the same result. Also, any transport with FDs in it becomes a magnet for every heavy weapon in your enemies army... namely because if they are rendered without a transport, 12" range is poor. even on a 4x4 board.
Wave Serpents with Chin-cannons and Starcannons (or indeed Scatterlasers/shuri-cannons), will put out a colossal amount of fire power, enough shots to start forcing failed armour saves. That, and are fast enough to prevent themselves being caught. These will undoubtedly win you the game. Support them with a Fire Prism, it gives you enough mobile firepower to wipe out entire units at a time
Now, Mephiston. No doubt, he'll choose the power that allows this absolute bullshit of a character to fly about. T6, W5 and a 2+ save make him a tough cookie to crack. make sure you have a defensive Farseer with equipment to deal with incoming warp powers. However... he is sinking nearly a fifth of his points into him. If you do your best to ignore him for at least 2 turns, and focus as much as you can on his troop choices, potentially by turn 4, you'll just have his more elite units to deal with.
Assuming this is the case, i would imagine his terminators (a 250 point unit, I would guess) will probably be Deep Striking in. If you can do enough damage to his initial forces (his troop choices!) you may be able to force to to HAVE to split up his forces to counter yours (this is where your mobile vehicles will come into play), isolating his units. Using your speed and his isolation, you should be able to draw angles to prevent maximum use of cover.
Mephiston will no doubt be flying about, there is a way to use this against him (especially in such a small game). He cannot join units. So, can be picked out. Concentrating weaponry on him can put him down, no Inv save is a major weakness for a model that cannot hide in a unit. However, focus on units that will down your skimmers, anything with a lascannon or missile launcher, be it a tank/man/whatever. Breaking the spine of his army means that no matter what his 'weapons' are, he'll struggle to use them.
Thus, I recommend:
Wraithguard w/ D-Flamers in a Wave Serpent with Brightlance + Chin cannon.
2 x 6 DA in Wave Serpents with Starcannon + Chin cannon
2 x Walkers w/Starcannons (Outflank these guys)
Fire Prism w/Stones and Chin Cannon
I don't have a codex on me, so let me know what sort of poitns are left over (assuming this idea interests you).
He's taking a whopping, great expensive character... so consider doing it back.
Asurman. Proxy him if need be.
Asurman is WS7, has 5 str5, ap2, Dire sword hits (both on the charge and being charged), has EW and a 2+ save... And, has the Exarch power to sit back and hide behind a 3++, whislt dealing one hit back at a time. Keep the Farseer there to keep him under divination/guide, re-rolls galore. He'll kill anything (I have kill Mephiston with him before). Oh, and 3 Eldar warlord traits.
6 Str8, Ap2 hits at WS7 and initiative 7. Splat.