Most times I tried the Four Kings pre-patch when there was almost no phantom to be found. And those I could find would for example immediately die, since they either don't have or forgot the abysswalker-ring. That's the kind of backup I'm having here. Yesterday I tried two more times, the first time getting the combined lifebar of them Kings about halfway down before I overlooked that the almost dead King in front of me was doing his AoE-blast. The second time I was spammed with those slow moving projectiles and spent most time running away from the massive damage they cause, just to be beaten to a pulp since all Four Kings had joined the party by the time I could dispose one of them. Maybe I should try to get some phantoms again, but so far the folks trying to 'help' me at the Four Kings weren't particulary helpful.
For Lost Iza you need a bow with plenty of arrows, the ring that allows to walk on lava and the repairbox (I assume everyone with some sense invests that 3000 souls early on
). Pyromancies are pretty much useless down here, since most of the enemies in Iza are resistant to fire. Magic works quite ok. The most annoying (I wouldn't say difficult) part was getting to the second bonfire and disposing all the t-rex-like demons along the way. Luckily they don't respawn. So you can take your time, look for a good sniper spot (some place where they can't reach for example) and take them out from a distance. Once past the lava I didn't find the area particulary challenging. There are some annoying monsters that cast acid wave, which breaks equipment pretty quickly if they catch you (hence repairbox or the repair spell
I don't know if you already did the Firesage Demon and the Centipede Demon. The way to them used to be a bit tricky due to the sheer amount of capra and taurus demons there. But since the patch I noticed the capra demons to be way less aggressive and some enemies on the way have even been removed.
Co-op is possible as far as I read but takes quite some coordination to get together in games due to the random nature of how the white sign works. However: each time a white sign is put down it is put on a different server (or something) and eventually it will end up on the server the other person plays on. The only problem now is that I'm still SL 67, thus a bit out of your level range