So I'm pretty new to Planescape. I invested the vast majority of my attribute points into wisdom and intelligence as I was advised to do, but my fighting skills are quite lackluster as a result. Early on in Sigil I just got completely owned by the necromancer in the mausoleum (the quest Nichiroji in the Gathering Dust Bar gives you).

I'm probably going to try and level up some more before I try that again, but in the meantime would anyone be willing to share some advice on this and future combat encounters?

Make good use of your companions to do most of the fighting for you early on. Morte is a great tank (and can be good offensively early on if you get him Ingress' teeth), and Dak'kon can really dish out the damage.

If this is the only battle giving you trouble just do other things first, gain XP, level up, find allies and better equipment before you return (or not, it's just an unimportant optional difficult sidequest).
If you have a mage in your party read the spell descriptions and use the spells wisely, Morte benefits a lot from temporary strength boosts for example.