Ok, so maybe I'm doing something wrong here, but it seems that any planetary mission with ++ or +++ is damned near impossible. I go in, wait for the fliers to jump out, attack the solo ship on the pad, pull it back from the base, 3 others jump in including a 'Conda and an FSD. Am I doing something wrong or are these missions completely borked?
Only some missions and only with some bases/settlements - yes, and mostly due to npc ships patrolling. I'm not a specialist in frontal assaults, I prefer a "sneak in" way, but if you are not in a wing you can try some common ideas:
1) all "kill n skimmers" missions - is better to search "wanted" targets in the settlements proximity POI's. Much safer and you don't need to deal with bounties on you.
2) destroy generator missions - yes, now bases have some point defenses, but as far as I understand, a good pack of dumb missiles can solve this problem as earlier days.
3) destroy defense turret - you can try similar approach. And think of generators that is simpler to shut down that can affect those turrets. Also, I know at least one base layout (large industrial low sec.), were you can "destroy" anti-ship defenses without alerting settlement at all (just scan respective terminal that have - surprisingly - "public" access mode)
4) scan something. Yep, this can be tricky - if you deal with high-sec bases or "military" layouts. In other cases you can normally at least approach the mission target terminal - may be by a chance it is in "public" mode. Anyway if there are no (or not dangerous in quality/numbers) defending ships than you have a very good chance of leaving the base in one piece (some recharges for your srv can help also). And as always I highly recommend to analyse the structure of base defenses and to make a plan of assault (or terrorist attack) before committing aggressive acts. Also this concerns the stage of accepting missions (for all those who like to stock them).
Scanning in hi-sec or large military layouts means, however, that you must do full assault. Military bases not only have impressive stationary defenses, but also usually good ncp patrol. Sometimes you can relog (if you accept this method) hoping to reduce an overall number of the ships on-guard. Also, once I've heard a brilliant idea (sorry, don't remember where) that I want to test once. The idea was not to destroy patrol ships, but to disable them in any means - rendering then harmless for your ship or srv (so supposedly there will be no new reinforcements). After that taking out the rest of defenses will take some time, but at least doable.
Beware of timing. This depends on base layout, but most of stationary AA defenses tend to respawn (at least if they were disabled by blowing up generators). Some bases can generate multiple (but limited) number of waves of skimmers.
Btw, I suppose that for some non-military hi-sec settlements a good knowledge of some terminals and their functions also can help. As can help settlement's maps that you can find in my signature (attention, spoiler).