Operations Engineer: Mostly harmless. Sets up an autoturret, which is also mostly harmless.
Tactical Officer: Web-grenades first thing, -fly -jump -run and immob. Then runs up and tasers you. Over and over and over. Don't let them get to step 2.
Sapper: endurance drain is annoying but mostly harmless, also has a ranged stun which can be dangerous.
Hercules Class Titan: Mostly harmless. Some knockback.
Gunslinger: Has a medium-duration single-target hold, and a high damage attack that can kill you, but doesn't use them much, thankfully. Don't let them run free, but you can generally afford to let them get a shot or two.
Operations Officer: OMGKILLITNOW. Do NOT let it get a shot off, because that shot will be Malta's trademark 30-second AoE stun grenade. This guy is why squishies hate Malta. (the sapper is why melees hate malta)
Zeus Class Titan: it's a bigger Hercules. Still pretty lame. Doesn't spawn on its own that I've seen.
Kitty Kat Nonsensewords: Like a gunslinger, except that he teleports. This makes him a lot easier to fight, because he does the dumbass trick of teleporting into melee with you, which is usually better for you than for him.
I've been doing Indigo's arc with Cinnamon -- she's outlevelled it, but not by so much that I can't set the diff up and still get whites. I got the 20% unique mez resist IO, and unlike the '30%' (actually 7.5%) IO, it has a noticeable effect -- I think it's still 20%, at least for now. Also, it was cheaper. *sigh*