Blitzcrank

Somewhat of a difficult matchup for Senna, as she is an extremely squishy champion with no escape with the exception of Flash. In the laning phase, you're going to want to play safe and patiently wait for ganks, poking with Q and not getting greedy for souls. In the late game, he is much less of a threat but a hook can still be lethal.

Nautilus

Nautilus is one of my least-favorite match ups, just because his kit consists of a lot of CC and provides a free shield that blocks most of your trade damage. Freeze waves and harass after he misses his hook.

Sett

Really annoying matchup. He basically walks up to you, gets a good E off and you're dead. Try to catch a W and get picks from there, and don't play aggressive in lane (Sett's early game is really strong with his Q/E combo, especially if his carry has a follow up).

Leona

Leona is somewhat annoying to deal with, as she is a tanky, aggressive champion with a gap closer/lockdown and loads of CC. When she latches onto you with her E, try to root their ADC and kill them instead. Trading is almost impossible because your auto-attack range narrowly beats her E range. Poke safely with a few auto-attacks and Qs. After 6, her ultimate becomes dangerous and winning 2v2 will be difficult. Team-fighting should come across as decent otherwise, so look out towards late game.

Jhin

Jhin has lots of damage and can win trades with Q and fourth shot. He also has an annoying follow-up and can hard-stomp Senna once he has items. Try to catch him off-guard while farming and poke with Q. Focus him in team-fights.

Pyke

Pyke's aggressive kit counters Senna and laning can be pretty difficult. He has lots of mobility, a hook, and heals pretty quickly with his passive even if you win a trade seconds prior. A Guardian Angel might be helpful later on, but try to abuse your range by poking and punishing when he misses his combo. Buy Executioner's Calling early on to counter his passive.

Swain

This match up is playable but very risky, as Swain's damage input in combination with his annoying root is almost deadly to squishy supports like Senna. Dodging his abilities is perhaps the best you can do in the early game and try to win trades. You do not win post-6 in lane, as his ultimate heals him for basically everything you deal with. An early Executioner's Calling rush may help.

Caitlyn

Look out for her traps and poke with Q. She has long range as well as a a net for escapes, but do remember that she is one of the squishiest champions in game.

Pantheon

Xerath

Super annoying poke and damage. You'll want to ask for ganks, but be prepared for a rough lane. Don't be surprised if your KDA is negative by 15 minutes.

Zyra

Zyra is always a safe pick, as she has insane DPS with decent CC and painful poke during the laning phase. Ganks can be useful against her, but do be careful. Poke with Q and keep the wave neutral, maybe go as far as to letting them push so you can set up for a gank. Build Mercury's Treads or Hexdrinker if she becomes a large threat.

Lulu

Really annoying. Can literally polymorph you and peel eternally for her carry. Try to make her waste her ultimate, because this lane should be even.

Samira

Has a spellshield and scales really hard, also has a dash and can burst you pretty quickly and get to safety almost immediately.

Vel'Koz

Even/major matchup; long range and decent damage. Look for opportunities to root and all-in, as he does not have any escape whatsoever. His abilities are sort of difficult to dodge, so be wary of his cool-downs.

Ashe

Recent buffs to Ashe have made her really annoying to play against, especially her W poke. I think that there are matchups that are much worse, but Ashe is really annoying to play against. If you see an Ashe with a CC support, you're in big trouble.

Brand

Brand is a mage support champion, but contrary to Zyra, his abilities are much easier to dodge. Sidestep abilities if possible and harass him once he misses something with your kit. He has high carry potential and can be very dangerous when fed, so an early Hexdrinker or Mercury's Treads rush is wise.

Morgana

As Senna, you are extremely squishy and a simple bind can allow her to win trades. Abuse the CD time on it when she misses and poke with Q. Her E disables the root from your W, but regardless you should be fine.

Alistar

You will want to play safe especially if their ADC is one with a follow up, such as Jhin or Ashe. Freeze lane and wait for ganks. Harass after Alistar misses his combo. He's in a relatively weak spot right now, but in the right hands, can be dangerous.

Nami

Trading against Nami can be very difficult because of her W, and this is especially dangerous when she's paired with an aggressive ADC like Lucian. After 6, she learns her ultimate, which is a very good disengage/engage tool. I think that it's overall an even matchup, just play smart and position well.

Thresh

Stay behind your wave and poke often with Q and autos. Be wary of his hooks. A good, aggressive Thresh will stomp Senna in lane, however playing safely under tower will deny them the option of being aggressive.

Yuumi

Just buy healing reduction.

Aphelios

Be aware of his guns! His early game (Post-3) is relatively strong due to the fact that he has very good sustain and can poke with Calibrum. However out of lane, I find that his worst time to fight is when he has Calibrum and Gravitum, because he misses out on a significant amount of AoE despite the good synergy between all of his guns. If he doesn't position well enough, it will be easy to win.

Bard

Bard's Q is relatively easy to dodge. Try not to stay too long in fights, because they can outtrade you.

Draven

Insane DPS and snowballing potential, but can be an easy push-over if he gets behind. Seize the opportunity to root when he missteps. Currently, the meta does not favor Draven at all, so he falls off very easily. Draven is pretty strong early when he plays aggressive, so don't greed for souls level 1-3.

Miss Fortune

Miss Fortune thrives through poking with Q, but does not have mobility early on. She becomes annoying post-6 since she can basically cast her ultimate any time and pick up kills. Kindly ask for ganks if your ADC is passive, keep the wave neutral and poke with Q. Harass once she missteps.

Kalista

Can be annoying to lane against if she is with a CC support. However she is weak early and does not win trades since you outrange her. Just position well; a good Kalista can easily play aggressive and pick up kills from doing so.

Shaco

Ezreal

Kai'Sa

Scales pretty well and basically does infinite amounts of damage over short periods of time late game. Laning against Kai Sa is the best way to gain a chance in winning the game, root and all-in whenever possible (reminder that you outrange her+she doesn't win trades without Pickaxe).

Zilean

Annoying to deal with early on, as his poke and stuns can be dangerous to play against. They may be easy to dodge, but a misstep from you can be a kill for their team. I despise this lane very much, personally. Keep an eye out for when he uses ultimate.

Soraka

Karma

Annoying harass and poke in lane, although you win late easily. Look for safe opportunities before collecting souls and trade with Q. Karma can be very annoying in lane but she ultimately falls off after mid-game.

Veigar

Lux

Her abilities are easy to dodge. Once she snares you, however, you're probably dead. Punish her after she misses Q/E because she misses out on a portion of her damage.

Janna

She's just going to peel for her ADC. Free kill and easy lane, though ward frequently to beware of ganks. She's annoying to deal with but an easy pushover.

Sona

Sona has a lot of poke, but she's also much squishier than Senna. Be wary of her ultimate's cooldown and you should be fine. She scales very well late game and can heal most of what Senna does late.

Rakan

Teemo

Braum

Kog'Maw

Jinx

Xayah

Fiddlesticks

Annie

Annie's stuns are extremely short ranged, which makes her easy to play around due to Senna's range. You can lane bully her pre-6, but be sure to have vision over their jungler at all times. Don't forget that once Annie hits 6, she becomes a bigger threat with Tibbers.

Corki