Well, to me there are two schools of thought to running AB maps. Others may have different ideas which may be better than my own.

The first is just a straight up medium rush...they're not very tanky, but enough to not fold instantly and do sufficient DPS to make what tankiness they do have last the entire map once you get a good deathball rolling...just make sure you send your slowest mediums first (bonus points if they're also the tankiest) so that the faster ones get to tank the later opponents as they outrun those that just finished tanking the previous ones, who still get to contribute their DPS without dying. I tend to favor this approach at the beginning of a battle.

The second is a combined arms approach. If the problem with A/D tanks is their fragility, send out a heavy first to tank with A/D backup so that their fragility isn't an issue because the heavy is getting shot instead. The problem with this approach is that any good heavy is going to cost 5 gears to deploy, and by the time you've got enough gears to deploy your A/D tanks (which should be a minimum of 4 gears, more likely 5) the heavy is over halfway dead (still able to pop defense and last a bit longer, but still). You can't really stack multiple heavies because of their gear cost, so you have to go one heavy, one A/D (maybe two) and then start over with another heavy as your first 'wave' is overrun. I generally don't like doing this until near the end of a battle when multiple enemy reinforcements aren't so frequent. (Also because at this point I don't have any mediums left and this is what I'm stuck with...I'm not going to NOT use my 5* A/D tanks in favor of more 4* mediums, after all.)