Best strategy for Nexus Siege (gathered from the time I've played the mode)

When attacking, you should attack as a group. All minions do is help push tower and prevent aggro when you don't damage the opponent, but in the long run they're not so important that you need to be constantly pushing all 3 lanes. When one lane is in danger of filling up the Obliterator, one or more of you can go to that lane and push a little, go back and buy an item or two, then go back to your group and continue pushing. Your opponents will likely be forced to start teamfighting from the start, letting your team successfully splitpush when necessary. You should also buy plenty of ballistas and teleporters, and place them in strategically good areas that'll let your team group together quickly while also being in such a place that it makes it almost impossible to get rid of it without taking damage from you or your allies. Overwhelm your opponents with numbers and constantly chip away at their towers, and you should be well on your way to killing their nexus long before 20 minutes. When defending, you should be both a group and be separate. Since you're not losing gold from minions, you can temporarily abandon a lane that isn't in trouble, and you can instead help overwhelm stragglers who insist on separation tactics. However, you should occasionally defend your lanes when necessary, and if this means separating for awhile then you should do it. Having one or two allies splitpushing defense should make it harder for your opponents to adapt, and by the time they've built the items to harass you it's already too late -- meaning: if it's Round 1 then it should be late enough that nothing they do will matter much time-wise, and if it's Round 2 and you used my strategy then they shouldn't have any time at all to do that much. For items, you should buy Storm Bulwarks when your towers need it (helps when your team is ganking a lane as well), and the other item depends on your own strategy.

These strategies can be both good and bad against Stackers (Veigar or Nasus). On one hand, pressuring them can prevent them from being able to be an asset for their team once it reaches late game; on the other hand, giving them free time to stack when you're NOT pressuring them can be fatal for your team, and it can backfire on you this way. If you have a Stacker on your team, they should prioritize stacking but should occasionally help overwhelm the opponent through numbers (especially Veigar with his CC, a well-placed E can destroy your opponents when they're going against you 1v3 or so). I'll admit that if your opponents know what your doing and know how to adapt to it, it could leave your team at a disadvantage. It also requires your team to actually know what they're doing, and for them to actually try to win the match (but also not be rage-quitters in case something goes wrong). However, it's extremely viable and should be often used...and to be honest, I'm not sure why I haven't thought of trying it sooner. In a mode where CS count doesn't matter and XP is given as quickly as in ARAM, you'd think we would've thought of doing this already.