In short, the best levels to collect as many targets of a certain type as possible, are (in descending order of magnitude):


Some comments:

• Level 23 is the only level that has targets of all types that are needed for the 'kill 1000' achievements, so it is theoretically possible to get all target based achievements by playing this level alone, but some targets would be under- and some overrepresented, which is why I suggest using other levels in addition.
  
• I recommend concentrating on levels 23 and 24 to get the general target count up, followed by levels 27, 29 and 22 to fine-tune the individual targets.
  
• If you just need to increase your number of hit targets to 10000 for the achievement and have a sufficient number of 'slow time' tokens, I recommend using map level 28 due to its simple makeup of having just 12 Hind helicopters and 15 Missile Bombers and nothing else.
  
• It is noteworthy that target hits from the USS Iowa do not count towards the 10000 missile hits achievement, because the Iowa shoots shells instead of missiles. I suggest using the 'Arsenal Prototype' ship and upgrading it to the maximum number of missiles.

The following table shows the various enemy targets along with their hitpoints, their worth in 'oil' and how many appear in each of the 31 levels. Using this information, you can choose the optimal combination of primary and secondary ship for the task at hand, as well as prepare an adequate amount of satellite effects (instant reload, electronic countermeasures, slow time) tailored to the expected opposition. Moreover, it tells you which map levels are the best to increase your number of specific targets, such as those needed for the achievements.


Summing up all map levels and grouping the number of targets in categories, one can see that it is impossible to get all achievements in a single playthrough, even if one destroyed every single target. Using the target frequency graphs from above, one can see that the level with the most Skyhawks is map 22, so the fastest way to increase your kill count of this target is to replay this map a few times.



As shown in this pie chart, the most common targets are the Frogfoot planes and Cruise Missiles, whereas the Skyhawk planes are the rarest of common targets needed for the achievements.

The following two graphs show the total number of targets in each map level and the total number of possible missile hits, which are useful in determining the best approach in getting the achievement to hit 10000 targets with missiles. Level 23, for instance, has the most targets and offers the most possible missile hits. Level 25, however, has very few targets while having almost as many possible missile hits due to the appearance of 12 Missile Bombers, which take 10 missile hits each to kill. That's why the graph labeled 'Number of Possible Missile Hits' is actually more important for the 'hit 10000 targets' achievement.


In order to speed up your ability to 'buy' and upgrade your ships, the potential maximum oil from typical moving targets (buildings are excluded) is shown in the next graph. The map levels of 22-25 are especially good for farming oil. Also, level 13 is the earliest level with an unusually high payoff if one manages to destroy the provided targets. As such, it might pay off to replay this map level a few times to rapidly upgrade your ships and buy new ones.