The first turn is basically the most important. After that the enemies usually get boosted shields. The terrain die weakens your first turn. They will help you deliver massive damage, but eliminating at least one enemy in the first turn is very valuable, which is a tactic the terrain die impedes.
If you do endless mode you'll find that the enemies are tougher than the bosses. Your run will usually risk ending once you fail to eliminate one enemy in the first one or two turns. After maybe 15 games i have done better every time without the terrain die. There may still be an optimal strategy that can exploit it though.