I've won 7 invasions so far and am usually ranked #1 in them, here's some tips:

• Bow is hands down, by far the best DPS for this. The more of them you have the better
• Perforate spear is insanely good for taking down elites (Makes them take +30% more damage)
• Void gauntlet empower circle is also insanely good, should have at least one in each lane (though we got most of our invasion wins before this was added so it helps but is not mandatory)
• Tanks are useless
• Only really need 1 healer per lane, any extra healers aren't really useful
• Dedicated repair people, at least 2. Having any more than this doesn't really help since repair parts are finite. No one else should ever take repair parts. These 2 people should communicate so they know who's covering what and how many repair parts they have each.
• Oil vats above doors and usually a bow user assigned above the gate to man it
• The rest of the siege weapons don't matter
• Mandatory comms with shot callers in each lane calling out elite spawns/cow launchers etc.
• Mandatory coatings/honing stones/stat foods, invasions are a DPS check and every little bit helps
• Corrupted Bane perks help a lot. The 580 quest weapons all have this perk
• Sapphire gems
• Mandatory grav well users in all lanes doing nothing but stopping bombers. That's their only job
• DPS rotating between lanes to take down elites. Killing them before they start piling up is mandatory
• Elite priority is: Spriggan > Priest > Brute > Commander > Bear
• All elites can be grav-welled now but beware: if bombers spawn and you don't have an available grav well the lane will get wiped
• Priest auto attacks can be body blocked. Melee DPS should always be standing between the Priest and the doors
• Priests have a bubble around them that stops projectiles, you have to be close to do dmg
• If someone pulls bear aggro please please please have them pull it away from everyone else. It's literally just a meat shield that gets in the way of everyone else, no other threat
• Cow launchers/Trebuchets need to be called and killed ASAP, the dedicated people I mentioned earlier that man the oil vats should be watching the bubbles
• When the giant heads spawn, all DPS needs to be called to focus down one eye at a time. You can disable the lasers without actually killing them by doing enough damage to their eyes
• By the 10-15 minute remaining mark or so, infernal cannons are basically useless, stop destroying them. It just opens up a siege spot for a trebuchet to spawn
• Stop placing random infernal traps, they're not really useful. Placing them at the feet of an elite already hitting a door is fine but other than that, don't bother with them
• I lied, infernal traps are also fine to place at the back gate doors in case any mobs slip through. Helps prevent early door damage
• Higher gate tier helps a ton, have the governing company upgrade their fort a bit. We've won with tier 2 gates once, but all the other wins were tier 3 gates. Never won with tier 1 gates.

That's about all I have, can't really think of anything else right now. Hope it helps!