Ok. Lets start this. One of the guys at my local plays Pac-Man and we play a lot, and by a lot I mean almost every single day so I have lot's of PacMan experiencie so this will be a real long wall of text.
*Neutral
When Hydrant is in play you must be wary of the timing of the water as a QA or Thunder might kill you if you have the bad luck of taking a surf while in the second QA or the summoning. So Pac.Man can be a little safer. Ww also can't camp Jolt as Pac Man con do his side b to make the pellet hit the jolt and recover 2% by eating (not that important but must take that into account because of timeout scenarios). So the best way to play the neutral this time is being patient. Pac Man lacks speed and movement and lacks those disjointed hitboxes we hate. Also, if Pac-Man is near the Hydrant and we feel like we can go in the bst option is to ignore the Hydrant and remember we can't safely use QA or Thunder. Pac Man can't really get in besides dash attack. Dash attack can cross our shield and has no lag so it's best to leave it unpunished and punish the next thing he does. Most of the time Pac Man might want to get his fruit so take your time to plan carefully how to aproach and steal it. Also be ware of Hydrant as it can deal massive damage and kill at high %
You objective is to get PacMan in the air because he struggles landing and you can do damage easily on the air. Kite around him until you see your entrance, then punish as hard as you can.
*Advantage State
Pac Man is a really annoying chrachter in ADV too because of a couple of things: Hydrant, Nair and Trampoline. I like to say "just a couple of hits, and fade out" that because nair and up-b come out really quickly and can disruspt our juggles and combos and be really annoying, Again: Be safe, don't overextend your combos, don't worry because PacMan has some trouble killing and in case of a timeout you can do the most damage if you play safe. I think the trick is to pressure PacMan enough so he starts doing risky things, that's the time when you can get in and do the usual Pika stuff.
*Disadvantadge State
I'll go to two possible scenarios. DIS with Hydrant and DIS without it. With Hydrant you might try to get out of harms way. Pacman can do some nasty damage if you get cornered or fall into his traps so the best is to avoid hydrant (or shield it if there is no shield break risk). Again be cautious if you want to QA because the water is stupidly trollying. If Pac Man corners you with a tumbling hydrant and he is near watch out for the grab, or whatever fruit he has charged.
*Combo Ability
Pika's combo ability is limited because of what I explained earlier with Tramp, Hydrant and Nair. PacMan also doesn't have too much combos if you can keep out of his fruit + hydrant setups.
*Punish Game
Pika can punish Pac Man if he want to stall in the air charging fruit, so use this to your advantage, beware of the hydrant though as it can hurt you, on the other hand if you can bait it you can punish pacman's landning really ward with f-smash and up smash. Also If you manage to steall the fruit you break the MU. You can shamelessly run away as Pac can't touch you. Sadly Pika's item throw is kinda slow so there aren't to many follow ups besides the ones with galaga, bell, and maybe melon. PacMan on the other side can punish greedy Pika's who try to extend a combo or juggle with Hydrant or some fruit.
*Ground Ground*
If there's no hydrant, go ham. Just watch out for dash attack and random fruits.
*Ground Air*
Punish air stalling, watch out of Hydrant Pacman can't land safe, exploit that.
*Air Ground*
Actually not bad Pac's anti air options aren't the best.
*Edgeguarding
Well Pacman can be very easy to edgeguard if they go low (shoutouts to Abadango and ESAM) If you hit the pellet with a jolt you can force him to do up.b and then just fall after the second jump into the trampoline and kill it. Don't go chasing him as Pacman's recovery has super armor so you might take a strong hit and even die at high %. While at the ledge something the Pac's like to do if you are waiting for it is to do a regular get up to an Up-b to get the trampoline and both in the air.
Pac can jump upstage and hit our extended hitbox with Hydrant, and he also can be very creative with the fruit (specially melon) again: be patient and everything will be alright.
*Killing
Here both struggle a little bit but nothing to hard. I think this is in favour of PIka as we play patient as Hydrant, Key, Back Air and jany setups can be avoided if being careful. On our side we can condition with jolts, Up throw thunder and some hard reads.
*Stages
Pikachu loves the mobility platforms give to us. Personally I like T&C and Smashville as the moving platforms can mess up with the hydrant and give us a lot more mobility and retreating options
*Top Level Matches
At EVO 2015 we all saw Abadango vs ESAM. Right of the bat, ESAM gimped at 0% Abadango. And then proceeded to go on his charachteristic aggresive PIka. This made him eat lot's of unnecesary punishes and when away he t-jolted camped on a flat stage, making it easy fodder for Pacman's fruit, even though we won ( barely ) the first game, Abadango won the set.
These two will met at SFW#9 here we see a different ESAM, even though his agrressive PIka is still there he plays a more patient game, going mostly for the true combos and not overextending. Fading in and out is great as he baits Abadangos grabs and Hydrants and gets a lot of punishes. Also everytime Abadango charges up a fruit in mid air ESAM egts a Punish. Eventually ESAM won this set.
*Quirks.
Galaga is PacMan go for combos fruit. Expect it at mid percents, also watch our for shield breaking shenanigans with galaga and hydrant.
Apple this is a Kill option for Pacman, it travels like our Jolt so watch out for that.
Melon it's a wall, Pacman may use it for stopping QA or to do some kill traps.
Orange and Cherry, Pacman fastest projectile they are just annoying.
Bell stunns and goes upwards then downwards. If hydrant is on the fiel watch out as water can move the bell and stun you.
Key fast and strong projectile, deadly in combination of hydrant and no platforms.
When Pacman is in the air and you bait the Hydraint feel free to F.Smash or Up-smash it. If pacman lands on it the hydrant will extend your hitbox and proably connect hydrant + smash combo on Pacman.
Most of your moves send the hydrant infront of you. Up smash makes the hydrante go like the bell (useful to grat a wall in front of you) and up tilt leaves it tumbling towards Pikachu (again, useful to wall)