A generally effective method is to have your surroundings memorized so that when Hitler comes too close to your position you can concentrate fire on him while running backwards toward your next safe spot. Because of this the player cannot rely on a corner of wall for cover for too long and has to stay on the move. Hitler's ability to rush toward the player makes him far more dangerous in this phase as his attack is devastating at close range.
Once Hitler is out of the mecha suit he gains a considerable boost to his speed. If you run too low on ammo when the second phase starts you will need to find more ammo while a signficantly faster Hitler chases you. Since the second phase moves much faster, it's wise to attempt to find ammunition caches to replenish your bullets between volleys during the first phase. This means making sure not to waste ammo and keeping enough distance that Hitler can't immediately run up to the player's face and shred them with point blank gunfire as soon as the first phase is defeated. Unlike most chaingun toting bosses, however, the player needs to be care during this phase to plan ahead to the second phase. As with any chaingun toting boss, keeping cover handy and poking out to fire on Hitler is the best tactic. During this phase Hitler behaves very similarly to Gretel Grösse in both speed and gunplay. The first phase of fighting Hitler is the Mecha Hitler battle.