Mission 4: Spam energy units, enough said.
Mission 5: Forget about that captain whatever to tell you use missile, IGNORE HIM. Build 2 outposts you will be fine. Stock up resources using outpost to generate manpower, when attack wave come push out tanks from both of them (if enemy only have air, use rifle to defend instead).
Mission 6: Watch the enemy units, and remember the ROCK PAPER SCESSIORS (practice in quick play hard-hard mode)
First ring: rifleman > scout > tank > rifleman
Second ring: MK II > air unit > sniper > ground units
Third ring: roman > sniper > tank > roman
Tank is the counter to mass ground units
MK II is the counter to mass air units
Roman is an excellent unit to absorb damage, allowing your more expansive units (tank, MK II, nighthawk, queen) to hail down the enemy.