The Darknik Dreadnought is the 5th Shadow Tower Boss. The Shadow Tower portal can be found through the "Atlas" in your menu. More information on this boss can be found here: https://trove.fandom.com/wiki/Darknik_Dreadnought_Mk_II
Ultra can be quite difficult to solo. Your best bet is to find an active club with high Power Rank members who can help you. Monday is Shadow Tower reset day, making it the best day to try and complete this.
I run each Shadow Tower on Ultra weekly. If you would like to tag along, shoot me a message and I'll help you out. As previously mentioned, Monday's would be best because there will be plenty of other people willing to drop in and help us melt the boss.
There's also an achievement for completing this on "Hard" difficulty that I could easily help you with alone.
Please let me know if there's anything I can do to improve this solution!