The last couple percents were just me grinding hours upon hours into multiplayer mode, hoping one of the last two multiplayer-exclusive powers would eventually drop at the end of a match. Luckily multiplayer is fun as hell, but I have to admit I eventually just started playing one-minute local multi matches with bots to try and improve my chances.
And yeah weapon fusion you gotta kind of build the weapon you want in reverse. Look in the massive grid of your hundreds of spare weapons, figure out what weapons combine to create the one you want. Then find out what weapons create THOSE weapons, and so forth, and try to mate the weapons with the powers/stats you want to carry over to create your desired weapon's ancestors. It's a crap-shoot a lot of the time though. Weapons will never get TOO awesome, so you gotta sometimes take one that has the power you want, and just forge a billion of the one you'd need to forge your awesome one with to get the final one you want, and just check each one. Will this one inherit it? No. Will this one inherit it? No. It's almost like Pokemon breeding.
In the end I rocked two very different weapons. One darkness bow that had an astonishingly long range and horrifying power for its dash/charge shots, where I'd routinely just KO dudes halfway across the map with single shots. I'd pair that one with the homing boost power to make it even more deadly. And the other one was a pair of claws that did great combo damage, but more specifically petrified the enemy so they couldn't escape. Go into battle with a clear strategy that matches your weapon while filling your power inventory with mega lasers to hold and pick off low-health dudes from any distance, and comet powers to nuke dudes nearby, and you will dominate.