Disable and Demolition missions are easier if you can manage to keep the enemies in the dark. It is tough enough to maintain a low profile without your demolition targets blowing up and alerting everyone around. Use the following trick to play solo easily through these two modes.
Disable Mode Taking Claymores for one equipment slot yields three placeable mines you can use. Place one at each objective as you proceed. With the finalone, blow up all your mines with the detonator. One mine is enough for these and saves you the time of holding SQUARE while he clips wires.
Demolition Mode You need C4 for each objective. Every map has three targets marked for demolition. You only get one placeable C4 per equipment slot. Place a C4 near the truck and proceed. On truck #3, hit the detonator to win. If you use the "Extra Ammo" equipment item, just use two C4 and plant the last one as usual. Note - Mission 3 (M3) has the chemical barrels that still need to be planted by using SQUARE, and they are impervious to rockets and grenades.