Much of the high death toll in WWI is attributed to outdated tactics. What would a WWII general have done differently, given the same technology of WWI?

Well, hindsight is 20/20 and all of that, so it is easy to look back and say what "should" have been done. This is all horribly hypothetical and not really sourceable.

Once the Germans entrenched after the Battle of the Marne there wasn't a lot the allies could do but follow suit due to their poor position. Things just kind of devolved from there, with neither side able to gain the upper hand...though not from lack of trying.

A lot of the great losses that we see are due to ill-conceived pushes against fortified positions, which account for the "outdated tactics" that you are talking about. Idiotic things like cavalry charges or bayonet pushes ate up men and resources for no reason.

What would a WW2 general have done differently? Depends on the general I suppose. The best case we could hope for after the Battle of the Marne would be that they simply did not throw troops away stupidly until...you guessed it...tanks. Yeah, I'm not ignoring them, sorry.

Our imaginary WW2 general would know what to do with tanks. Don't trickle them into the battlefield, keep them a surprise until you have a hundred or so, mass them in a single schwerpunkt supported by your not-horribly-sacrificed troops and push them through no-mans land. Use them as a shield to get your troops amongst the enemy, then use the surviving ones as fire support.

Once a section of the trench is secured, move fast against enemy artillery.

Since the Germans only started producing tanks in response tho the appearance of Allied tanks on the field, a well-planned surprise like this should break the stalemate.

Source: My own ass, since this question is so hypothetical :)