Guardian and bosses
Big bosses usually lower their head enough for melee characters to hit and you can also hit them at their feet/hands. You can always use a Scepter if you want to go range.
Also, give the staff a chance. It has performed pretty well for me in PvE.
as an guardian vs bosses, you can stay close range , use hold the line, heal from staff tank for few seconds , you should be able to take 5-6 hits then retreat and use range weapons
I’ve only did the Shadow Behemoth in terms of really big bosses, and I found I was running around too much to really do much damage. I apparently did enough, because I got Gold, but I spend a lot of time supporting (not that I really mind that).
At level 30 and up, an aegis heavy build with retreat and renewed focus (and maybe a mace+focus) will let you block and negate damage from 7-10 attacks in a row. I laugh at bosses until I run out of aegis and dodge (that’s rare, btw).
I solo champions and group events with Scepter/Shield and Hammer.
Pretty easily, actually.