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Up close Leon loses to 11i mids, so you have to make the reads and hold them. Anything slower though you can use jab strings. You really need to use jabs.
To deal with jabs you have 33p and 2p.
Then for lows you have 3p, 6k, 6p, or 33p depending on the speed.
Most won't start with lows, but if you're playing someone like Ayane, her 2p will beat everything you do. You really need to know your match-ups.
3k, 8k, 46p, and jabs are his best spacing tools IMO.