For completing the campaign on difficulties other than Predator, I suggest you exploit the infinite feral abilities. By jumping from objective to objective, both Instincts and Predator can be completed in half the time. Simply charge a jump while in feral mode and skip over or through all enemy encampments. With the exception of snipers, the enemy AI has incredibly bad accuracy and you should emerge relatively unscathed.
Avoiding unnecessary engagements will save you a considerable amount of time on both Hunter and Rookie. I completed the expansion on Hunter difficulty in about an hour and a half rather than the usual three to four. Skipping through the jungle does pose one risk: snipers can and will pick you off even on the easier difficulties! Clear them out before moving on! Moreover, some objectives require vehicles to ram through barricades. Pay attention to your objectives before skipping through an entire level.
The only potential problem you may encounter is protecting Doyle on Predator difficulty. Identify the enemy patterns (as they vary) and keep trying. Regarding the Instincts boss fight, once the regular enemies have been dispatched, take cover behind the helicopter and shoot whenever you spot an opening. Once he jumps down from his perch, rush him and repeatedly push B. Each time the boss is hit with a feral punch he will be momentarily stunned. With infinite feral, simply button-mash B as he will never recover.