Relying on a large personal tank in typical pvp engagements is often a losing proposition.
Using speed, range, or electronic warfare scales better against larger forces.
Giving up on tanking completely though is a mistake. While a personal tank works well in small gangs and fails in large ones, remote repairing is actually the opposite.
The more ships you have the more effective remote repairing becomes. The target gang size for a remote repairing is typically about 10-30.
Once fleet size gets above a certain point ships being killed before they can be repaired becomes more and more of a problem.
Battleship Spider Tanking
Battleship spider tanking gangs can be some of the most effective fleets in EVE.
With a decently sized group you can take on large amounts of firepower while losing few if any ships.
Even if a battleship spider fleet gets wiped out, it is fairly cheap to replace due to the insurance payouts.
Especially compared to other advanced tactics such as HSLR which rely on T2 ships, battleship spider fleets are light on the wallet for how effective they can be.
When fitting a battleship for a spider gang the key attributes are armor amount, armor resistances, lock time, sensor strength, and capacitor.
The starting point for any fitting should be the tank, meaning 2 1600mm armor plates, 2 energized adaptive nano mebranes, a damage control, and 2 large remote armor repairers.
Additional tanking modules and rigs are very beneficial as well, keeping you alive longer.
After this a sensor booster or two with scan resolution scripts will bring down your lock time to a sufficient level, allowing you to quickly start remote repairing gangmates.
Once you have your lock it's important to keep it, so at least 1 ECCM is highly recommended. Keeping someone locked is worthless if you run out of capacitor, so a heavy capacitor booster is required as well.
Notable modules that are not needed on a spider tanking battleship are any local repairers (you rely on your gangmates to tank you, slots should be used for hp and resistances instead), and any shield tanking modules (it's key for everyone to have the same tank). Another item that may not be required is a microwarp drive (since everyone needs to stay together speed isn't required) - however check with your FC/XO first.
Flying a Spider Battleship
When in combat your number one concern should always be the group tank. Of course you should be putting ewar on opponents, utilizing your drones, doing damage, and everything else, but these should all come after the tank.
If someone else is taking damage then they will be sending out a "need armor" fleet broadcast. Simply right click on this broadcast and select "lock target." As soon as the gangmate is locked then you should put one or both of your remote repariers on them. You can exercise some judgement here.
For example if the gangmate is still in shields and not taking much damage then you can hold off on repairs a bit, but be sure to keep a close eye on them and not to forget.
Similarly if they're doing fine with incoming damage then you can stick with only one repairer, keeping the second for if they start taking more damage or if someone else starts taking damage. Of course be careful, you will be locking both friendlies and hostiles, so don't accidentally start shooting a gangmate or start repairing a hostile.
If you start taking damage yourself then be sure to broadcast "need armor" quickly.
You should broadcast when you start taking damage, not when you enter armor.
This will give everyone plenty of time to get you locked and get repairers on you before you really need them.
If you're being primaried there's not a whole lot for you to do, just focus on offensive actions, be sure any hardeners are on, and rely on your gangmates to keep you alive.
Once you have repairing down then the next step is being able to stick together as a gang so that everyone will be inside of remote repair range.
This is most commonly done by having a common point that everyone stays close to. This can be something like a bubble, a cargo container, or most often a gangmember.
Commonly a gang leader will just have everyone stay close to him, so it's a very good idea to have the gang leader on your watch list, making it easy to quickly pick them out.
When staying close to something your best bet is to rely on the "keep at range" option, usually at 500m. Approach or orbit can work, but keep at range will work better in the vast majority of cases.