I would go for the Falcon's Talons for UN4 with my partner also equipped with it as well, and the scythe for UN5, but as far as being tougher than UN1 and UN2, they are. I seen a few people on the leader boards who soloed UN2, and he did it with the Talons, and if you have patience for certain moments in UN2, it will pay off for you. I would also say the toughest trial of them all is MN15 even with a partner as much as MN8 can be painstakingly difficult to solo, but I seen very few people take on 2 marbuses, a handful of fiends and soldiers with rocket launchers, and it's possible though. Two of my favorite trials are MN07 and MN09. They're fun, and fairly easy if you know exactly what you're doing. And lastly, I noticed you didn't mention UN03, cause anyone can make it look easy, especially if you know how to stun-lock all the clones.