Well both are very different and should be approached differently. So I'll begin with invasions. First and foremost, when invading throw all notions of honor out the window. You have forced yourself into the world of an unwilling victim. So if they are already being attacked by another invader, gank them. Apart from that situation you'll be fighting a mainly uphill battle. If you fight a war of attrition with the host you will lose. I recommend a heavy weapon that can end them in just a couple of swings. The next thing you'll need, is terrain you're comfortable on. Invade in places you feel comfortable invading in, that you know very well. The last few pro tips I could give you on invasions, would be to have gear and stats for many situations. Bombs, arrows, and even a spell or two if you can squeeze it into the stats. Use the enemies especially in a gank, it's ok to back very far off up the level and take potshots at the host as they battle their way through the level, this gives you every benefit, with no detriments. Lastly, be patient, there's no need to rush things, wait for opportunities like the host being split from his phantoms, or the group being swarmed by enemies to strike. Plan your strategies around these crucial areas and moments and you'll be very successful. Now for duels. The most important thing for duels is min/maxing your stats based on the weapon you want to use. Duels should also be taken slow, see what your opponent will do, and look for patterns in his attacking. Learn to parry bait, and most importantly, be as unpredictable as possible. That should be about it.