I: Trenches
The trenches are pretty tough. As the name suggests, half the time you're down in dirt walkways with enemies up above you. You can go up ramps to those upper levels, and shoot down at the ones below you. The trick is to find angles where you can shoot or blast the enemies on the opposite level without them reaching you. Also watch for the floor spikes - there are quite a few of them.
This has the wizard coin level - get the 50 coins and you can now be the wizard. That can be fun!
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