I mostly play gold so I can only really speak for that difficulty, but if I'm hosting I generally have no issue top scoring while tanking everything. I rarely even dip into my consumables. Take damage/DR/defense damage on siege pulse. The first few waves you'll demolish everything with it, then you'll want to hold back a bit while you're getting swarmed.
Use hex shield for all damage. 6b just gets in everyone's way, while 6a helps mitigate the damage lost from 6b of fitness. It will still be more than big enough to block damage from one direction, and with all durability in fitness have more than enough strength to last, helping you ward off turrets and ravagers, while stunning, blasting and DOTing everything in the immediate area. Use a shotgun (GPS if you have it) with an omniblade so you're heavy melee actually hurts and use hex in between. This will whittle down the elites swarming you with surprising speed.
No turret. It's great team support but doesn't benefit you enough even with 6 ranks, and we're assuming selfishness here

I forget what I took in the class ability for rank 4 but it's not so critical. 5a and 6b definitely, to exploit geth weapon damage, and boost power damage which you rely on most.
Fitness is all durability. This is how you stay up. Destroying everything requires you to be on your feet. Having hex shield out will mostly replace the overall damage lost from 6b. 15% of all damage for 75% health and shields is more than worth it and you'll still have great damage output.
Ultimately you can be the tank your team needs while still dishing it out. Remember to fit in pulses even when you've got brutes and banshees all over you. Get your timing right and you can have pulses back up before you get wrecked. Use this especially when you have team mates with FIRE. When everything is glued to you, setting off a fire explosion will be extra devastating. Use them to close distances, knock back an atlas during their missile animation, and get attention off your friends. Like I said, after the first few waves it'll be all about timing.
Use any gear that gives you shields. I like the straight up shield booster, because 30% more shields on this guy is all you really need, and is a lot. Don't waste your cyclonic mods. Strength enhancers since you're gonna melee a lot, and adrenaline mods to cover the map quickly, are also most helpful on my opinion. Get creative with ammo.