This one in particular. What worked for me is building a small force to take and hold the tiberium field up north, fortifying there then sending out an assault squad by the lake to destroy the enemy base. It's difficult, but doable. If you can't figure it out, look up a video walkthrough on Youtube.
Spoilers ahead! Okey,I'm stuck in the Sarajevo mission, where you need to get the engineer into a structure in the center of the temple. I've reached the point where you blow up a wall, and on the other side thre are allys assaultnig the center base.
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