The Saboteur

I got this achievement and the 'Mad Bomber' Silver Perk in conjunction with each other.

NOTE 1: The 'Mad Bomber' perk requires you to destroy 5 german vehicles with dynamite in 300 seconds.

The way to do this is to 'aquire' a german vehicle, any will do but i recommend either the Sturmwagen (Nazi Car) or the Bauer (Nazi Lorry) as these are guaranteed to blow up straight away (unlike the armoured vehicles).

When you aquire a german vehicle, take it to one of the resistance garages and store it. You will then be able to select the german vehicle you stored and it will appear at the garage ready for you to drive.

The timer for the 'Mad Bomber' perk starts when you first destroy a german vehicle with dynamite.

All you have to do it select the german vehicle, plant the dynamite and wait for it to blow up.. don't worry about killing the resistance member standing at the garage, he cannot be killed in this way.

When the car blows up, speak to the resistance member again and select the same german vehicle (it will cost you 10 contraband to repair) once repaired select the same vehicle and repeat the tactic above another 4 times.

You dont need to worry about the 300 second timer, as you should easily destroy the 5 vehicles before the timer runs out.

NOTE 2: The timer cannot be seen on the screen while playing the game, to see the timer you have to pause the game (which also pauses the timer) and look on 'Perks > Demolitions > Silver (II)

NOTE 3: Don't worry about causing attention to yourself, in the saftey of the Resistance garage, the germans will not respond to the sound of the explosions. (or they didn't with me)

As for the 'Demolition Derby achievement, you can count the 5 german vehicles you destroyed for the 'Mad Bomber' perk towards the 50 needed. It is then a matter of preference on how you destory the remaining number of vehicles.

NOTE 4: Any vehicles destroyed as part of an ambient freeplay is also counted towards the 'Demolition Derby' Achievement.

Admitidly this whole method is slow at the start, but if you were having difficulty getting the 'Mad Bomber' perk then this method i found to be the easiest and with the vehicles you destoryed as part of the 'Mad Bomber' perk adding to this achievement.. it's win win.

The easiest way to do this would be to steal one of the nazi's AA guns and then gun every thig down. it will raise the alarm and soon the vehicles will be pouring out onto the streets.

the AA way

Another way I done it was head to the countryside between Paris and Le Harve, found a AA gun near a spotlight. Took out the spotlight and surrounding troops, there was no other guards nearby. Jumped in the AA gun and starting shooting Zeppelins down, yes they count as a vehicle. Good thing is they respawn so just take them out as they appeared. My alarm did rise to level 2/3 but no troops turned up to start shooting at me. Didn't take long to shoot down 50.