I've done this sort of thing before, and someone else has even just done it again recently, but I figure I might as well go over useful spells again, but this time for invasions specifically. Yes, it does end up a bit different, with different tactics. So, my standard setup has these spells: Homing Crystal Soulmass, Snap Freeze, Crystal Soul Spear, White Dragon Breath, Crystal Hail. Great Farron Dart, Farron Flashsword, and Soul Greatsword.
Homing Crystal Soulmass: I always cast this first, unless the host or their phantoms are already on me when I spawn in. It's dead useful for getting some room at the start of the fight, or landing a cheeky hit right at the start. It's also brilliant for nuking blues that are too thirsty, by facing away from them until they get close, starting a cast of Crystal Soul Spear or White Dragon Breath, and then doing a 180 and insta-killing them. The soulmass will stagger them, and then the other spell with finish them off before they can roll. If you're not locked on, people have to be closer to you to trigger it, meaning you can start casting a spell, lock on to someone, and have the soulmass stagger them so they can get obliterated by the other spell. You can also cast this if you're low on stamina, and then face away from the person you're fighting. They'll be expecting you to do a 180, and will stay back. This gives you plenty of time to regen some stamina.
Snap Freeze: Useful for two things: creating an area of cover that most players won't enter, which buys you time to estus or cast a slow spell, and it also works to punish people who are just holding their shield up and standing in a small area. If you're standing on one side of the cloud, there will also be a point where it becomes completely opaque, so a Crystal Soul Spear can catch the person you're invading by surprise.
Crystal Soul Spear: Does lots of damage. Try to catch people off guard when you cast this, or when they're busy with something and can't dodge it, since it'll almost never hit someone who's ready to dodge.
White Dragon Breath: I only started using this today, but it has paid off multiple times already. Its casting time is slightly different from Crystal Soul Spear's, which helps catch people with it, since the casting animation is the same so they roll at the wrong time. It's also good for hitting people who are on ladders, since it can travel up and down walls. The fact that it hits really hard, for only 25 focus, is also pretty good.
Crystal Hail: This will probably never kill anyone, even if they're low. What it will do consistently though, is make them panic roll. Use this if you need to estus, or if the person is really low on health, where any hit will kill them. Just spam Great Farron Dart, and hope for a hit.
Great Farron Dart: I use this when I'm not being given a lot of time to cast, but want to land ranged hits. I also use it when the invasion has been going on for a bit, and I'm starting to run low on focus and ashen estus. It can also be good for spamming down people who are turtling behind a shield, since it's so cheap. Use this in conjunction with Farron Flashsword for damage at a low cost, and swap between them to keep people on their toes.
Farron Flashsword: Good for fighting straight sword users who can't take hits well. If they're trying to R1 spam you down, turn the tables on them and do the same thing to them. It's also good against slow weapons, but you have to pace your hits and be careful. Try to land one hit at a time, and then roll through their attacks. Also good for when you're at low focus. This is basically a straight sword, so use it like one. It does rolling attacks, and also running attacks, so if you mix regular attacks with the other two ways, you can get some respectable damage. Just remember that it can be parried, so watch out. If you think they're about to parry, switch to Great Farron Dart, and surprise them.
Soul Greatsword: Good for catching people with its reach, and does good damage. You can combo it with Homing Crystal Soulmass to deal some solid damage. It's also amazing for catching people through walls, since they don't expect it. If they're on one side of a thin wall, or about to round a corner, you can cast this and smack them for a free hit. Try it on the stones around the Mausoleum Lookout, where people hide to avoid the arrows. You can usually land a hit or two doing this.
This isn't all that's useful, but it's 8 spell slots, so it's what I use. Other spells to consider are Homing Soulmass (some people swear by it, but I don't bother with it), Farron Hail (good for landing some low damage hits for a low cost, like Great Farron Dart), Great Soul Arrow (good damage for a moderate cost), Soul Stream (use this with the obscuring ring and binoculars to land long range hits on people), Old Moonlight (if you land both the sword and projectile on a charged hit, it will out damage a Crystal Soul Spear), and Affinity (same as Homing Soulmass).
Some tips for invading:
Stamina regens even when casting, except slower, and only for part of the cast. Use this fact to get the most of your stamina. Instead of waiting for full stamina, wait until you're close to full, and start casting. Once you've learned your cast times and regen rate, you can get significantly more out of your stamina than you would expect.
A good setup for estus is two regular, five ashen. You'll almost always be better than the people you invade, so you'll take fewer hits, and you'll need a lot of focus.
If you're looking for fast invasions, invade as a mad phantom, and bait people into a Homing Crystal Soulmass and Crystal Soul Spear combo. It'll kill most people in one go, letting you leave right away.
Try to avoid letting fights last for a while, since you can burn through a lot of focus. The exception is if you can kill summons without using lots of focus, in which case you're golden. Just ambush and nuke blues as they come in, and you can stay in good shape for 15+ minutes, if the host is cowardly enough.
Don't use estus until the host has, unless you're being ganked. You're going to have a lot less than they do, so they'll always be at an advantage. However, sometimes they will treat the fight as a duel, and won't estus until you do.
Some hosts are total cowards, who will try to wait you out. What I do, is put on a video on my other monitor, and watch that. Running around the area exploring is also a fun option, since you find some neat stuff you hadn't come across while playing through there normally.
If you a blue gets summoned, try to make sure they spawn in on the host's side so you don't get caught between them. Focus on the blue unless the host presents a good opportunity. Blues will usually rush in for a kill, and they're often not very experienced at fighting mages. Use this against them by throwing any dirty trick you have at them, and let them run into your casts. Kill a blue, and you get two of each flavour of estus, both citrus and blue raspberry. This is also one reason why you take two regular estus, so that you maximize your gain. If you abuse this, you can take a fight that's going poorly, and turn it around.
Corridors are your friend. If you can force people to only move forwards and back, you've cut out a lot of room for them to dodge.This makes it easier to fill the area with spells, to ensure that something hits them.
If you're being ganked, get to some enemies to form your own gank. Let the host and his phantoms fight them, and then jump out behind them while they're distracted and teach them the error of their ways. This is a good opportunity to see how hard your spells hit, since they won't be paying attention to you. A soulmass and Crystal Soul Spear will again make short work of one of them. Invading in areas you know well, with shortcuts and hiding spots, will be quite helpful.
While people advise against trading, your damage can be good enough to make it worth it. If you can tell the person's in one-shot range, try to bait them into a trade. They might not be thinking about how much damage you do, but pure sorcerers can put out more damage than people give them credit for. Sure you might take a 600 damage hit from their greatsword, but the 900 damage Crystal Soul Spear that kills them at the same time will matter a lot more.
If the person you're fighting is super passive, don't bother with any high damage spells. Instead, use low cost ones that you can spam, and try to rack up damage over time. They can roll forever, but you can only cast spells for 10 minutes, unless you're careful. Another option to combat this is to just leave them alone. Walk further into the area, and stand out of sight. Put on a video, read a book, check some websites, whatever. They'll eventually get bored of waiting for you to come back, and move on. Then, when they aren't paying attention, insta-kill them. They deserve it, and the following point down.
If you end up fighting someone using a weapon with Quickstep, counter it with Flashsword. You will hit for around the same damage, and if you time it right, you'll get counter damage as well. Just try to get a feel of when the person is likely to attack, and punish them. Do two swing combos for a bit, and then once they get comfortable, smack them with a third swing. If they let up, consider using a soulmass spell, and maybe Soul Greatsword.
If you are in a fight that's taking some time, and haven't used one of your spells yet, try to bait them into it. They might not expect something new to come out if it's been a few minutes, so it'll be easier to land it. You can also do this with a trick instead of a spell. Basically, try to not let the person know everything you can do, until they're dead. If that spell shares a casting animation with something you've already used, that's even better.
All in all, it's a fun way to PvP, that'll test some skills that playing the usual melee builds don't. Just keep your spell order consistent, learn to change spells as fast as possible without missing the one you want, and learn your cast times by heart. Do this, and you'll start a nice collection of tongues, vertebrae, and anything else you care to farm.
