The goal of the tactics below is to inform the GM what behaviours produce a good enjoyable game for all (point 2), what behaviours they should adjust (point 3), as well as (point 1) to try to make them aware of the fact that they have a different role at the table as the GM (as judge, as arbiter of fair, and as the interpreter and controller or rules) in addition to the role they had as a player (to have fun, to try to not ruin the fun of others)

It's unclear from the description if this person is new to GMing or not. This may be a case of the person GMing in the only manner they know how to play. This will be easier if that is the case, but even if not, my approach in this scenario is to use all of the following:

1) "Catch him in the act" ... When an attack roll that used to miss now hits, ask why. Accept the answer, but make sure to question every time. The idea here is to make the GM know that they are expected to play by rules (for the purpose of asking that the GM follow rules, what actual rules doesn't matter, just that they need to play by some rules).

2) Reinforcing (positive) Feedback ... When there is an encounter that runs right, talk it up. Say how fun it was. Say how tense it was. Point out swings that appear to happen naturally and how they produce story. The goal here is obvious - to encourage more of the good stuff.

3) Adjusting Feedback ... (which it sounds like you are at least attempting) When things are not right, tell the GM - after the fact is fine - you're not out to embarrass or insult, but to get the GM to be better next session - but make sure to say both why the encounter was not as fun as it could have been (the on-the-fly adjustments made it feel like deus ex machina, rather than real tension, with a real chance of failure) AND most importantly, what would have made it better (if the uber-whip-dude had been a little less tough, the start of the fight would have been easier, but the end of the fight would have been a tense touch and go affair)

Different playstyles

You virtually say that you and the GM in question do not want to play the same type of game. The answer, as with so many things, is to talk to them. But the topic should be how to get into the same style of game.

If the GM is playing a mostly tactical game and expects most of the drama and fun to come in handling tactical situations, then trying anything else is going to be at best unfun for him and probably won't work well. Remember, if he has spent a lot of time planning a battle, balancing it against the party, possibly custom making maps, he is going to be invested in seeing that battle happen and understandably reluctant to a non-combat resolution. There is nothing wrong with a mostly tactical game, as long as everyone agrees that is what they are playing.

You seem to want either a wide open sandbox or at least a game with very light rails enforced invisibly, and you seem to want most of the action to come outside of combat. This is a valid style of play (and one I prefer), but it requires a GM who is good at improvising (not everyone is) and willing to set aside any set pieces he may have spent a lot of time planning. Not all GMs are even capable of playing this way (at least not until they have practice doing it). Even ones that can, may not find it fun.

Finding Middle Ground, or not

So, the answer, depending on relevant situations may actually be for you not to be in that game when he is GMing. It sounds like you want a very different style than the GM is providing. He is not providing you fun, but shifting to accomodate your style may very well ruin the style of fun he enjoys.

Less extreme than that, you could both consider shifting some towards the middle ground. He may agree to hide the rails better and be ready to hand you information, and go out of his way to make some of it useful for the upcoming battle. But he may ask you in exchange to focus your solution finding to creative ways to prepare for the combat rather than really avoiding the combat he has invested time in preparing.

For some examples of that, think about Wizards in D&D. They shine best when they are facing an enemy with particular strengths and weaknesses, and have had time to prepare for those in their spell list. You could be the one to learn the information about what they need to prepare for. The Witcher in that series is similar. He is a powerhouse if he loads up with the right potions and oils and traps...but needs to have an idea of what he is facing ahead of time to do that. A savy player could spend a fair bit of the game time researching the creatures to know what to prepare for (or looking that info up online). Your character could be like that, researching how to prepare and then doing the preparation. Batman is also like this.