Score Maniac
100 Million Score
Theoretically, you should get this by the end of the game, as the score will keep going up until you run out of lives, but if you still haven't got it by the end, you can just start a new campaign on Easy and wait until your score reaches 100 Million.

True Angel
All civilians saved in one level
The easiest way to do this is by replaying the first level since it is the shortest and most harmless one as the only real danger comes in the last 3 waves as more trucks will rush towards your town, so before they outnumber you make sure you clear the area of any remaining soldiers on foot because that's how most civilians die after you free them from the trucks; try stay as close as you can to the town and you should be fine.

Lets hit the big points
99x multiplier
Just like the overall score, your multiplier will go up over time as you score combos, but the difference here is that your multiplier will go back to zero if you die, so perhaps the easiest way to get this is to replay the campaign on easy and just stay out of harm's way

Armed and Dangerous
Maxed out the weapon level
As you get rid of more bandits, different items, special weapons and upgrades will drop; just keep collecting the weapon upgrades ones until this pops up.

Sniper
Finished a level with over 25% accuracy
At the end of each level, the accuracy will be displayed on the scoreboard; keep your shots focused on the enemy targets and avoid wasting bullets when there aren't any enemies around and this shouldn't be much of a problem

Big boom
Killed 5 enemies with one Nuke
With your nuke special weapon selected, try to attract as many enemy vehicles as you can around you and launch it.

Arsonist
Turned 5 enemies into flames with one Napalm
With your napalm special weapon selected, try to make as many enemy vehicles as you can follow you before firing it.

Invincible
Full armor collected
Enemies will often drop armor, so make sure you collect it before it disappears; collect enough to fill the armor bar in the left corner and this achievement should be yours.

True survivor
50 kills in Survival bonus level
Survival Bonus level will unlock at the end of location 1, just make sure you get at least 50 kills before you die as the entire point of the challenge stands in the fact that you can't regenerate HP, so once your health reaches zero, it's over.

Ultimate Frenzy
50 kills in Frenzy bonus level
Frenzy Bonus level will unlock at the end of location 3; get at least 50 kills by the end of the challenge and this achievement is yours.

Rally Driver
50 checkpoints collected in Racing bonus level
Racing Bonus level will unlock at the end of location 2; just pass through at least 50 checkpoints and the achievement is yours.

Ganger Slayer
Harvester boss killed
The Harvester is the first boss you'll fight and it can be defeated using only Napalm, so the quickest way to do this is by making the Harvester follow you in straight lines and using Napalm to cover your back, and once that is done you just have to make sure you get out of its way before it charges on you.

Renegade Slayer
Tank boss killed
The Tank is the second boss you'll fight, which can be defeated by placing mines on its path, and in a similar way to the Harvester before it, the quickest way to deal with it is to load up on mines and just stay ahead of him, placing mines behind you as he follows you.

Mutant Slayer
Mutant boss killed
The Mutant is the final boss and it can only take damage after it has been hit with an EMP, so before jumping right in front of him, make sure you fill up on EMPs, ensuring that you have at least 40 of them and then just using a combination of EMPs and weapon hits to take him down.

Airline pilot
15 seconds of airtime in one level
There are some levels where you can use ramps a couple of times in order to get this, but every level has some sort of hill that should give you at least one second of airtime if you take it on at full speed, so no matter which way you choose, just repeat the same process until this pops up.

Full batteries
Recharge to full hitpoints in Recharge Area
Once all enemy vehicles are eliminated in a wave and you still have time until the next one comes, the recharge areas will become available; just stand still in one of them until your health is fully regenerated and this should unlock.

Check out the extended guide and a lot more about Wasteland Angel at https://gamedragons.net/viewtopic.php?f=14&t=163