Their life expectancy wasn’t good.
From 1943 onwards the fighter sweeps and low level strikes into Europe saw the German flak troops attached to the Wehrmacht and SS ground units really start taking a terrible toll of the Allied fighter bombers, particularly before they got their tactics right and truly mastered the art of what we now know as “CAS Shock & Awe”, ie: stay low, stay very fast and use the terrain, use your wingmen to spot flak positions for you, check high ground etc etc.
The Flakvierling 20mm and Flakanone 37mm guns were absolutely lethal and so important were the fighter bombers to the Allied efforts that all steps were taken to ensure the Allied ground units knew to expressly target anti-aircraft units wherever they were encountered. The US and British armies found this an easy sell, as a shortage of fighter bombers due to losses meant spending longer under the 88s and German tank guns. They were only too happy to agree to take them out as a priority.
You also saw in Saving Private Ryan the Germans bringing up a 20mm flak weapon and using it (very effectively indeed) as a ground assault weapon, which was a common occurrence. Worth noting in the film that the flak gun crew all died. This was a common occurrence too. But they were extremely effective in ground fire support roles. This merely made it an even easier sell to convince the Allied ground units to wipe out the German flak units everywhere they came across them. The Allies did use the four barrelled 0.50 cal M-45 mounts (that earned the grim nickname “meat grinder”) to great effect in the same way, both trailer mounted guns and those mounted on the half-tracks. The truck mount M-20 never saw service in WW2.
Allied fighter bombers would fight in pairs of pairs usually. British and American doctrine was for the lead to do the attack run whilst his wingman suppressed the ground fire. In the days before radar and with a dangerous mix of new recruits and older veterans manning the guns it was often hard to see and target the fighters, who were excellent at using terrain to get fast and low before the flak gunners could see them. You hear the rising pitch of snarling Napier Sabres and the whistle of its turbochargers so you can hear a Typhoon pair moving into a dive - but you can’t see them. Then from your left, or your right, or right behind you, there is a deafening WHOOSH of rockets and you entire position is obliterated in a hellish cacophony of fire and explosions.
Allied pilots loved the rockets as one salvo was famously likened to a broadside from a heavy cruiser and the effect was fire, smoke, deafening noise, sheer pandemonium and intense, stunning concussion. Ideal for subduing flak batteries in that you didn’t even have to hit them or their ammo stocks to stop them, a salvo in the same rough area would be enough to rattle and scatter all but the hardest and bravest of crews. The rockets would go in and the precision killing was done by the 20mm guns in the following-up Typhoon’s wings (or the eight fifty cals in the Jug’s wings etc.)
Meanwhile the lead would be rolling in on his attack run on the original target.
By the end of the war the sheer numbers of veteran experienced troops that were lost by Germany meant that many or most flak troops would be kids or the youngest conscripts. Strategic flak batteries were manned mainly by Hitler Jugend by 1945 with kids as young as 12 running shells and calling out height markers. The field flak units remained a thorn in the side of the Allies and special care was taken not to leave any flak units still operational after a strike if possible. So, often the strike fighters would bravely make another strafing pass to make sure. This is one reason so many fighter bombers were lost on CAS missions. Of course, by 1945 the war was lost and the poor kids and junior conscripts manning the guns knew very well that their chances of surviving what was left of the war were slim.
There are many documented cases where suppressive fire from fighter bombers caused their greener crews to panic and take cover, which obviously is not ideal performance from the flak troops you are depending on to keep the Jabos off your artillery guns or refilling station etc. Even the mere sight of a Typhoon, Mosquito, low level Thunderbolt or the much-hated P-38 would cause panic amongst the flak crews as their spectacular rocket salvoes truly terrified the Germans.
The brave crews stayed and fought and in many cases won, only to likely be killed later. The inexperienced or poorly-led crews would panic and take cover as they knew what was coming when they heard those aero engines. The crew armour protection was pretty much non-existent for the towed pieces save for a flimsy gun shield that many crews actually removed to save weight. Even the Mobelwagen and Wirbelwind tank-based SPAAGs offered precious little protection for their crews. The Coelian (which did) did not arrive in time to see service in WW2.
Even so, as I’ve mentioned, the Allies continued to lose many fighterbombers to ground fire right until the war ended. I believe they had a considerably higher loss per sortie rate than the heavy bombers high above.
It must have been a truly grim job. To have the strength to stand and concentrate on getting the gun ready to fire, laying on clips of 20mm or 37mm when every bone in your young body is shrieking at you to take cover quick as the lead pair of Typhoons roll in on your position… truly incredible. These were kids who had seen the charnel house destruction, the body parts and the fiery deaths of their mates on other gun crews whenever the Allied Jabos fired their rockets. Hard to imagine how strong and brave they would have to have been to stand their ground knowing what grisly fate likely awaited them in doing so.