Assists I use
Frogadier: Ignore Eevee, Frogadier is where it's at. He's able to apply long range pressure and set up on enemies.
Diglett: Similar to good ole' Froggy, but he will launch the enemy in the air. In Duel phase this is very good, as you can combo into Up+X then Up+A to Izuna Drop the enemy for ALOT of damage.
Field Phase
Field Phase can be hit or miss depending on who you're fight and how good they are at zoning. I have a very hard time against zoners like Braixen, well... ESPECIALLY Braixen. Shadow Mewtwo also seems to have an answer for everything. Laying down traps correctly is manditory as Sceptile I find, his homing attacks don't really allow him to dash in too well unlike some characters, unless he's in burst mode in which case he'll crush the enemies guard and put you into Duel phase.
Throw out Aerial Y first, because it takes 3-5 seconds for the leaves that make enemies flinch to come out. Depending on where the Opponent is, throw out A,A and hold the direction they're running to create a wall of seeds, place yourself behind the seeds to make a wall between you. You can then either chose to do Aerial A to throw more traps down, or approach depending on the situation. Overall you want to try and approach once Aerial Y starts shooting out the red leaves; you should be able to get a grab on them if the leaves are nailing them down, switching phases.
Another thing to note, if you use A,A, if the opponent is hit by them (Sometimes they won't be paying attention as Aerial Y will be right in their face assuming you're mid range), then Side+Y,Y will combo into it and the backflip kick will knock them into duel phase.
It's either that, or hope to land a side+Y,Y while dodging a projectile, which is pretty easy to do against certain enemies, especially with all the traps out, giving you plenty of opportunity from the distraction.
Duel Phase
I actually find sceptile to be pretty decent in Duel Phase. Upon entering Duel phase, I'll instantly do A,A to lay down seeds if I initiated it and the enemy is getting up from the attack. If it was them who initiated duel phase, dash backwards to try and setup. Then Aerial A to throw more traps down to create some space. Down+A is fantastic as it gives you life back and the opponent can't block it, but it can leave you pretty open if you just throw it out repeatedly, so use it sparingly. You want to play keep away as long as possible unless they're better zoners than you -glares at Suicune and Braixen-. A,Y is a fantastic spacing tool for those trying to creep through the traps, it's got very long range and comes out quickly, you can also combo into anything, including Y,Y from it, which I'll get to why it's good in a second.
If they're better zoners than you, you want to get in as quick as possible. Mix it up between blocking/dashing, and using Back+A (hold) to get in. Remember during Back+A (The vine swing) you can hold R (Block) to cancel it into the stationary vine, this is good to stay above certain projectiles, you can also do any aerial attack out of it, X is good because you can combo out of it with Y,Y,Y which leads to many mixups when you dash cancel it or do something which I'll mention soon (seriously, it's useful).
If you approach from the ground rather than Spidermanning your way in there, XX is my main tool of approaching, but Down+Y is nice too as it is a quick poke that moves you forward slightly and combos into Forward+A.
As mentioned in an earlier post, getting life back is one of Sceptiles gimmicks with Giga Drain (Down+A). Looking at what was just said, we can combo into it in several ways... Leaf Blade (Forward A, the Second hit of XX and the third hit of YYY, the large ranged blade) sets up enemies at practically the perfect range to land a Giga Drain. So using what was just mentioned...
Enemy trying to get in? Have you hit them with A,Y while trying to keep them out? Combo into YY, then throw out Down+A. Big damage and you get life back.
Just used vine swing and hit the enemy with Aerial X after it? Combo into YYY to throw out LeafBlade... Down+A. Get some life back.
Close range and you land XX or YYY? Combo that into Down+A.
Now, this isn't 100% foolproof. They can jump out of it or move/dash out the way. But it's pretty safe. If they dash forward, it puts them within grabbing range. If they jump, you can pretty much Shield before most attacks will touch you. Dash back, you're safe. Mixing all of the above with counters, Counter Attack Dash Cancels, and grabs, can make Sceptile pretty tricky.
If you do either XX or YYY, if they block the first X or haven't gotten hit by the second Y, dash cancel them (Block+forward or back). If you dash backwards, it leaves you safe and you've created room to setup traps. If you dash forward, you can either go into another combo, you can grab the enemy, you can charge a counter attack, you can do A to hop up and do whatever you want, drop down on them with X, throw out a quick Y, throw out some seeds with A... It's up to you, mix it up, keep them guessing. You get the idea.
He might not have the best frame data or power behind his moves, and has some weaknesses, but he's got plenty to keep him going. I think because some characters are pretty damn strong (Gengar, Suicune, Braixen, Pikachu etc), it makes Sceptile look pretty lame in comparison. No doubt the game will be balanced though and he'll either look better in comparison and/or get some buffs. That's just how I play as him though or what is going through my mind as I play as him, it seems to be working out pretty well anyway.