Shooting the Invincible

Hello friends,

A friend and I have been working on the development of a new board game for the past few weeks, and we've actually made something that our playtesters feel is very fun. It's a game where 5 players work against 1 (maybe two?) in a cooperative manner. The board is set up with a tile laying system where each tile counts as a space for movement. the objective of the group is to obtain a certain predetermined number of objects from the tiles, while the objective of the one is to reduce their "health" until 0. The group has to work together to prevent this.

4 of the tiles currently represent locations where the 1 player may place their units, and there is a mechanic where the players are allowed to play a certain card (current mechanic) to "shoot" one of them. The one player is then forced to withdraw that piece to another of those respawn locations for their next turn.

Unfortunately, in current playtesting, it has been determined that the group has little perceived incentive to shoot since the 1 player gets enough actions to move fast enough to make up the distance. It's key, though, that the group gets a way to hold the bad guys at bay. Is there anything that any of you can think of to help fix this mechanic?

Other details to consider:

-The game board is currently 6x6. expanding it would probably increase incentive to shoot, but would also make the collecting aspect harder on the group (though that's a minor issue I suppose)
-The players get a certain number of actions per turn, then draw a card to use on the following turn (similar to pandemic, but all cards are more immediately useful)
-The lone player is currently slated to get two turns per rotation around the table (hence why it could be expanded to two). At present, the lone player is a hair too powerful in terms of balance (why we need a "keep them at bay" mechanic) and the good guys are having a hard time planning around those turns (which is actually kind of a good thing: keep them on their toes, right?).
-The tiles can rotate as the game goes along, but the placement of individual tiles is unlikely to change.

Any help that you can provide will be really appreciated.