The max number of active skulls is 2.
On the first level (A-1) pick the skulls Blind (cant see you health bar) and Famine (dropped weapons have much less ammo). With these two on the level is still a walk in the park.
You pick the skull by tapping them. Just keep your distance from the enemy where possible, let your team mates do some of the work and if you see your shield deplete (like a little burst of electricity on your character) just run to cover.