Can anyone give me some help on the Sheik, Charizard and Fox match ups?
As with most matchups, don't get grabbed. In these MUs especially, you can get set up for tech chase situations very easily.
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Sheik:
Sheik's grab game speaks for itself. Her needles & tilts can stop poor approaches, so play it safe & be smart when attacking.
She can't approach well; therefore, securing a lead against Sheik will play heavily in your favor if you can hold the advantage. She's easy to combo, resembling that of Lucas, a slower falling Fox, Charizard, etc. Ledge guard = bait out needles & double jump aerials, then grab ledge and punish the special fall end lag.
Her OoS game isn't amazing,
especially if you cross her up on shield (Bair / Nair have bad hitboxes behind her & Dair is slow). Be cautious of needles & ledge hop aerials when recovering. She can decimate Lucas once he gets offstage. Pressure on sheik is mostly easy comparable to other cast members.
Be smart & safe similar to fighting Marth and you will win this MU.
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Charizard:
Charizard is combo food. He should take at least 50-80% from a hit, if not being 0-deathd. The massive hurtbox is unfavorable to him and most of your attacks should not whiff. Hard pressure and not dropping combos will mostly win the MU.
His flame thrower & Nair are the biggest moves to watch out for. Dsmash, Upsmash, Ftilt, Dtilt, and Jab are the other tools that Zard will use to space out and wrack on damage. They're somewhat laggy and can be punished really easy. Avoid being in range of Charizard while he's trying to land or out of hitstun. Fight Zard horiztonally and bait out his moves then punish.
This MU is really easy if your combo & bait game is on point
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Fox:
More on Fox later. I'm still working on mastering this MU. If the Fox player is amazing, this MU can be tricky. If your skill difference resembles M2K's Fox vs Neon's Lucas at LTC2, then Lucas will win. Combo game on Fox & ledge guards, while not free, he has tons of option coverage and has all the tools to combat Fox when played correctly.
Avoid pre-emptive jumps and bait Fox into the air. This applies to ledge recoveries as well. One shine spike on your double jump and you'll die 9/10 times. Always try to tether ledge FIRST if you are in range. If you get hit out of tether, you'll retain your double jump and UpB's. Double jumping first sets up a ledge guard opportunity. If you're forced to use UpB, most times you will end up getting gimped / ledge guarded.
Don't fight him airborne, this should all be done either by using anti-air moves (dash attack, Utilt, Ftilt, Fsmash) or trapping landings and THEN going in with PKF, Fair, Nair, or Dair. His Nair (and most Nairs in the game) will always beat PKF.
***Dash attack, while you should not use this as a crutch, is one of my favorite moves in the Lucas v Spacies MU. This move sets up for BRUTAL combos, such as all your smash attacks after 50-70%, all tilts at most %'s, grab, and all aerials. Go into debug mode and look at this massive hitstun that spacies are in from this move. You have lots of time to confirm off of one dash attack. play around with your combos.
At 0-20%, go for magnet -> Bair / Nair combos. It will easily grant you a quick 13-20% and start to chip away from their crouch cancel.
Avoid Fox players mindlessly Nair / shining & recklessly upsmashing you and absorb / dodge as many lasers as possible. Upthrow - Uair is easily SDI'd and Uthrow -> Bair won't kill until over 110% with good DI. The highest I believe Lucas dies from this setup, is around 130%.