There is quite a wide variety of mass attack strategy in the game, which is one of the reasons I enjoy planning so much. The downside is that a good attack plan does take time to prepare. Here is a non-exhaustive selection from the top of my head:
[EASY] The pick and send!
Idea - Post up on polls "let's attack Scallywags lads, pick and send on this tick!"
Pros - All the organiser has to do is pick a time. The burden of "picking targets" is then split among the individual members, so normally 2-3 ticks warning to the alliance on pols is all that is required.
Cons - Can only work with who is online at the time, depends on people refreshing. A lot of the time people are busy and just post "pick me a target please" which places a burden on the organiser to work to a tight schedule. Also usually only provides a single wave and single member for each target, so it's relatively easy to defend.
Top tip - To dramatically improve your pick and send attack planning, get each sender to choose 1 real and 3 fakes. Suddenly, even with only 4 people online, you've just covered 16 targets rather than 4, for relatively little additional effort.
[EASY] The train!
Idea - Load a bunch of people on a single target and then plough a bunch of players / latecomers behind the first senders.
Pros - Again, dead easy to plan. Just say "4 people send on the first tick, the rest of you follow one at a time". Easy to spy and check the target for defence.
Cons - Trains result in a much smaller crop yield than more challenging planning.
Top tip - The balance between how many people to put on the first tick and how many behind is actually non-trivial. There is no magic answer to this, in a rank 1 alliance attacking a rank 2 alliance, the typical sufficient size of the head of the train was 6 members. But when attacking an alliance with far fewer online, 2 may do the job. Try to ensure the head of the train uses a good route combo - Striker+SA is perfect. Also - to sneak in one extra land grab for the alliance - have an arsonist send a tick in front of the main train. Often the defending alliance would rather block the tail end of the train rather than the sneaky arsonist in front (just make sure the arsonist can survive a tick of self cover from the target).
[MEDIUM] The single wave.
Idea - Pick a time and date in advance for the attack to get maximum members online. This will be a compromise between the most members YOU can get on, and the fewest members THEY can get on. Get at least 2 different routes on as many enemy IDs as you can.
Pros - Normally, you can cover an entire alliance quite easily. It is scientifically proven that "divide and conquer" is the best technique in war! Quite challenging to defend if unanticipated by the enemy alliance.
Cons - If spread too thin, may be easily defended. Also, as it is only on a single wave, there is greater risk on staying on an attack that has defence, since you have no idea which defending mobs are real or fake.
Top tip - Make sure that each target can't self cover. I know it sounds obvious, but it isn't. If you are expending 2 mobs on an online SGT player, make sure you can actually outflak the SGTs/yobs.
[MEDIUM] The multiple wave.
Idea - Here you spread the outgoings over more than 1 tick. There are many different ways to do this - for example you could have the second wave one tick behind the first (covering last tick) or three ticks behind the first (bypassing all defence on the first wave). The first is good for units that fire R/M only (e.g. RPGs) and the second is good if your alliance is trigger happy and will send all in on the first wave.
Pros - If you make the first wave fake, you can actually draw out a lot of real defence. This means that you force the enemy's hand into realing the fakes so you can get lots of land on the second wave without a fight. Even better - the first wave will actually TELL YOU whether the defence was real or fake. Nothing like hard evidence to tell you what to do with your second wave.
Cons - If spread too thin, may be easily defended. Latecomers will defend the second wave with unknown reals/fakes. Also, those that can self cover themselves can defend both waves. Also - your alliance has to be able to send 2 mobs at 30 minute intervals (not always easy on your phone when at work).
Top tip - Make the first wave a convincing fake, with perhaps eta 2 stealth mobs thrown in. If they take the bait, you'll be landing all over the shop.
[HARD] Permutations of the above with rushes.
Idea - Not only do you launch eta 5, but also include rushes after defence has been sent. This could include thiefs, RPGs, harriers, anything to kill those offliners/UC members. To really make a great attack, players should be sending 100k geos to different targets and then boosting one another. This gives the defenders FAR less time to make an assessment of where to defend, and even if they do, you can boost low eta troops to assist in real time.
Pros - This is a personal favourite of mine, because it is so flexible. You can change targets on the fly, as the defence is sent. Spotted a large robotics player defending real on your last tick? Have a harrier boost my good sir. Thank goodness he wasn't tied up on another target by sending eta 5... Also the defending alliance can think "great, we've got that target covered, let's consider the next one..." only to suddenly receive a game-changing hooligan boost on the target they thought was covered.
Cons - An absurd amount of micromanagement for the attack planner (for large waves). Works brilliantly for smaller waves or resistance rushes though. Also there are some defences that you just HAVE to anticipate in advance (e.g. last tick dog defence). A minor con is that the low-eta unit players get less land than the eta 5 robos/dragons.
Top tip - Draw out the defence first, then slam an arsonist rush onto an unsuspecting target with support from harriers/apaches/PoMs a tick behind. These kind of rushes are SO hard to defend against, they really are an absolute nightmare when you are committed to defending against other incoming.