Simply add [] to those names like
<input type="text" class="form-control" placeholder="Titel" name="levels[level][]">
<input type="text" class="form-control" placeholder="Titel" name="levels[build_time][]">
Take that template and then you can add those even using a loop.
Then you can add those dynamically as much as you want, without having to provide an index. PHP will pick them up just like your expected scenario example.
Edit
Sorry I had braces in the wrong place, which would make every new value as a new array element. Use the updated code now and this will give you the following array structure
levels > level (Array)
levels > build_time (Array)
Same index on both sub arrays will give you your pair. For example
echo $levels["level"][5];
echo $levels["build_time"][5];