You always check once for each enemy, no matter how large, gargantuan or colossal he might be. On the many tests issue, i wouyld rule that once the enemy has been able to stop you, that is that, you gotta try something else that round, like going around, but cecrtainly not through him again.
if that would be the case, i would rule that you loose the movement cost needed to enter the square, doubled unless the -10 penalty is applied, so each attempt would cost you 10ft of movement in optimum situations.
Note that a tricky Dm can easily make the ground not a plain and easy tumbling ground without mch effort, heightening the DCs by 5 is an easy task and that usually means charging is not an option too.