Hum. I'm doing Level III missions with a Myrmidon, and I've only got to structure just once (the first time) - the trick is to get all unfriendlies to get a lock on your ship before launching the drones. After they lock on you launch the drones and tank the hits while the drones do all the work.
5 Ogre Is can really chew through cruisers in no time.
Then again, I prefer long-range railguns to up-close-and-personal blasters - I've put in weapon range extenders and a drone range augmentation rig - drone range goes to 55kms...
I run level IVs with a Myrmidon and can pretty much tank anything without effort.
I'm running a passive shield tank all SPR IIs in lows a mix of LSE II Shield recharger II in mid. Autocannons in highs as grid and cpu allow. Give it a try.
Check my standings 8+ with 7 corps if you doubt the shield tanked Myrmidon.