If you put a creature you want to use at a later date in an encounter, how could I save that creature near the end of the battle without sounding cheaty. E.g. The evil mage has only 1 hit point left and gets smacked by a barbarian, but wait he has an immediate interrupt which teleports him away. This gets more complicated if the creature gets surrounded so can't realistically escape.
It sounds like you have a very acceptable fire-focused intention. I suspect one of the things that is complicating matters is the fact that:
The Mage Wizard can select 2 Encounter powers per level, so that offers some variety. It helps that the DM lets us waive the restrictions on the number of powers we can use per day. I think that's why he makes the monsters so strong in each encounter, so they don't get wiped out easily.
Therefore, much of the normal optimisation advise, which assumes that you're holding to the normal rules starts melting away as the vicious circle of buff and counterbuff begins. (I faced this problem in a "by the rules" game when the DM reacted to the party's increasing optimisation by ramping up monsters, which caused us to optimise more, which...)
From a pragmatic perspective, save ends effects suck. While much of the game is well modelled, there is precious little balance to save ends effects, and the pendulum swings back and forth: standard monsters have little to no defense, but elites and solos become effectively immune as the game design progressed through the monster manuals. It takes a very deft touch in monster creation (if you're creating monsters "from scratch" to respect player agency in the inflicting of status while simply not going "nope!" to either them automatically winning or to them automatically being ignored.)
I, personally, have always enjoyed the more controlly-type controllers, and so my wizards, druids, psions, and invokers have focused on debuffing and forced movement. So long as you rely on effects that are more difficult to shed (either being end of next turn or encounter long) then you can focus on being to reliably land them, rather than inflicting sufficient debuffs to the monster's saving throws (that'll only be countered by the next monster) to maintain the debuff. The same thing is true in the other direction. I've played paladins who granted +9 to saving throws by smiling. This led to the DM completely foregoing the use of save-ends effects until the DM and I agreed to voluntarily limit that feat to a +5 bonus.
My recommendations are:
Nothing is as powerful alone compared to a party that is designed to work together.
Stop focusing on solo optmisation. It's a trap. Instead, try to make sure the party is designed to work together to achieve your desired requirements. Everyone will have more fun, and you're unlikely to bear the brunt of your DM's nerfing alone.
Have a side conversation with your DM: Explore what debuffs he's comfortable with.
Boundary setting is important. If you have a chat over coffee as to what he considers reasonable, you won't find the powers nerfed in the middle of a game. Set up, describe, and agree upon expectations for your character's capabilities such that he knows what to expect (such as to provide you maximum Fun) with the minimum of unpleasant surprises. As 4e is very much combat-as-sport, the joy is in the execution of plans within a chosen narrative (yes, story matters, to provide a need and justification for mechanics) than it is finding unusual solutions to the DM's prepared set-piece battles (many other systems are far far better at simulation).
It's very hard to alter characters in midstream without a retcon. Be honest and do a proper retcon, don't just knudge.
A character is the combination of her parts and their interactions, not just the parts alone. If you're changing a character's rasion d'etre, be honest about it, and change the character completely to fit your new requirements.
Well. There is much advice and hints and tips and tricks to be learned. But I will keep on task, and I will help you in your specific situation. You can ask more questions later, relating to any other aspect of running a game that you want.
The start of the ambush begins as the players first start on their way. You describe their surroundings, what kind of path, or road they are traveling, you ask them to describe how they are traveling, what things they bring with them, etc... because obviously as the GM you should know what your players are up to, how they are dressed, how they prepare themselves, etc. This lets the players describe to you their preparedness without them knowing they are doing that in preparation for an ambush.
Example: The players are just packing up their camp on their second morning on the road between two towns. You describe the path to them as muddy from the nights rains, and the scattered trees as glistening with rain drops, the breeze is sharp and slightly cold, and the sun is still rising over the horizon, You ask them to describe how they dress, how they prepare, because after all - the journey awaits. The warrior describes to you how he is wearing his armor and cape, but that he puts his bow in the wagon because he fears it getting dirty on the mud if he walks with it, and the wizard explains how since his robes are now soaking wet, he switches to his traveling gear (which incidentally, don't have his secret pocket full of alchemist fire flasks). etc etc.....
Here is an important part of the ambush, you want to check if maybe your players characters can detect the ambush, but you must do it in a way that they do not feel they are about to walk into an ambush. You want them to roll their perception (spot, listen, smelling, 6th sense - whatever) without knowing its for an ambush, and also after the roll, they should be feeling safe and unsuspecting.
Example: You describe to the players how they spot the carcass of a dead deer on the path ahead, and ask them to roll their perception. You generously describe the deer and its surroundings to those with higher rolls, and mention that it poses no threat since they can clearly see the animal was killed by a hunter who took its antlers. The players who may have initially been concerned with a dead body, are now calm and unafraid. And you have their perception rolls for the scene.
Something is not quite right. Some of the players may start to be suspicious, this indicates that the players have caught on to the fact that something may be afoot. Preferably this happens after you have already got everything in place, if not - you need to act fast before the ambush is too expected. Regardless, this is the point that you will be at right before The Reveal.
Example: A player may ask you "Will anything interesting happen here? Why are you describing to us this boring forest as we walk by?" Or "Dude, this is totally an ambush!" (okay, I don't have amazing examples)
This is the moment you have been building towards, it needs to be grand and really make your players feel surprised. Regardless, once the ambush is revealed, your deception is pretty much over, and combat (or fleeing) will begin.
Example: Maybe an arrow suddenly flies past them, narrowly missing them, signifying the start of the ambush. Or perhaps the shouts and screams of a dozen Kobolds suddenly surround them from all sides as they charge forth from the forests concealment.
On The Table
About what you do physically: Only pull out the map after the ambush has been revealed. Until then this is just a normal day traveling the road, and you describing the wonderful scenery. Based on their perception rolls and how they describe their characters, you will judge their preparedness (including their initiative rolls) to see if they can draw weapons, or prepare anything else for the ambush.