Havn't tried bringing a merc to uber trist yet.
Would it be worth to invest in a uber merc for killing speed as a smiter or other decent uber character?
And if so which is the bis gear?

not really a point to making a merc op, you can run ubers with a very budgeted gear smiter without an issue.

If i were to suggest one, for just the fun of it, you could use a few different items depending on your budget. seeing your fg, these should be pretty cheap

A2
helms:Vamp gaze or Ral'd andy.
armor:Eth fort or treachery. you can get fade to prebuff and put on fort, but that's just a time dump imo
wep: insight with good ed and +5-6 crit strike/ebotd

i would say that BO would help a lot, but mercs still die fast so not really a point in the investment unless you are able to borrow from a mf char you already have.

I don’t belive it gives any killing speed if you have good smiter, but if you want your merc to survive then something like this works :

- Eth Kira’s ” UM ”

- Eth CoH

- Eth Tomb / Eth Upg Hone Sundan

Worth investing? no
Is it fun? yes

A5 Merc Azurewrath(res jewel) and any sword like Head striker, Plague, Hustle, Grief, and my favorite Last Wish etc, Arreats(Cham) and Fort(along with Treachery pre buff).

I tend to get him as close as 300 LR and 300CR vs über Mephisto.

Oh, you can use Lawbringer instead of Azurewrath if you don't use Last Wish(Life tap).