First of all, I want to state that the “Tiger’s level” isn’t really a thing, or at least nothing relevant. There is the character’s “effective Druid level,” which determines the Animal Companion’s benefits per The Druid’s Animal Companion. These include bonus HD, which do wind up increasing the Animal Companion’s “level,” but that’s not really relevant to the question. Animal Companions do not simply “level up” with their masters; they just gain bonuses (including more HD) when their masters level.
How the rules work:
He counts as a Druid 2 for the purposes of his Animal Companion. A Druid 2 cannot have a Tiger; he must be at least Druid 7 so that he “counts as” a Druid 1 (after the −6 penalty) with respect to the Tiger.
As a Ranger, he would not count as an effective Druid 7 until Ranger 14, at which point he would get a regular Tiger and only the 1st-level benefits listed in the table, i.e. Link, Share Spells, and 1 Bonus Trick.
Personal opinion: this is dumb, and it should be the other way around
The Ranger gets an Animal Companion so far behind the Druid that it’s basically good for not much more than scouting. Meanwhile, the Druid gets an Animal Companion that can be nearly as good as the Fighter. This is stupid because the Druid without an Animal Companion is stronger than the Ranger and his own, better Animal Companion combined. The Druid is one of the most powerful classes in the game, but even though the Animal Companion is very good, it’s not even the primary reason (or even the secondary reason, for that matter) the Druid is so good (it would be #1 spellcasting, #2 Wild Shape, #3 Animal Companion).
So I usually houserule that Ranger’s get the better Animal Companion (starting at 1st and at a level equal to what a Druid usually gets, rather than halved), and the Druid gets the weaker version (if he gets an Animal Companion at all, if I can even be convinced to allow Druid; I don’t outright ban it but I try to help players who have character concepts based on Druid express that same concept, if possible, with another class).
What you might do in your game
Anyway, back to your Tiger. As a 7th-level character, this is what a single-classed Druid could get; I’d be inclined to just allow it for the sake of not upsetting things. I’d let the character use both Ranger and Scout levels for it, and I’d let him have a “full-level” Animal Companion, so a regular (effective Druid level 1) Tiger is fine for his character.
In a couple of levels, assuming he continues in Ranger or Scout (or something else I felt was appropriate for advancing it), the Tiger might start to get better (i.e. effective Druid level 3, he gets 2 bonus HD, +2 to his Natural Armor bonus, +1 to Str/Dex, another Bonus Trick, and Evasion, and then so on as he continues to level).
I might ask the player to take a feat for the benefit of using any class level, rather than just Ranger or Druid levels, to count for the purposes of his Animal Companion.
If you don’t have a Druid in the party (and it sounds like you don’t), these are pretty easy changes to make. If you do, you may have to let them both have the “strong” Animal Companion, since the Druid player probably doesn’t want to lose his pet. That’s probably fine: it’s not like you were going to fix the balance problems around the Druid with this change anyway.
Can you dispel a magic effect you can't perceive?
Yes, as long as you can locate it somehow. If you can't find it somehow, then no. This reduces down to the related question: Can you target a target that you can't perceive? And the answer to that is a qualified yes. Yes, if you can land your dispel magic on the desired target somehow.
As we'll see, your example situations don't allow you cast the spell yet, because you haven't targeted anything. But first, let's look more at how targeting works.
Targeting spells
To target something, you need two things per Targets on PHB page 204 (unless the spell's own description lifts one of these, or adds new requirements):
The ability to choose the target
A typical spell requires you to pick one or more targets to be affected by the spell's magic.
And dispel magic does require the ability to pick the individual target:
Choose one creature, object, or magical effect within range
A clear path to the target
To target something, you must have a clear path to it, so it can't be behind total cover.
This isn't a problem in either situation you're asking about.
So in order to target something, you need to be able to individually choose it. To be able to do that, you need to know that it's there and where specifically it is. You often gain that information by sight, but sight is not required, only knowing the location of your target is required. This information can be gained by many other means: hearing, touch, divination magic, etc.
Now, about that qualified “yes” above: can you target something you can't perceive at all? Only if you have enough knowledge from something other than perception in order to correctly target it.
For example, if a god granted you divine intervention and whispered in your ear to say “the invisible wizard is hiding behind the third barrel in the south-east corner!”, then you'd be able to target that invisible wizard without needing to use a perception ability of your own. This choice would be “I target the invisible wizard right there, behind that barrel.”
As another example, if you had a note from a wizard that said, “cast dispel magic on the centre of the wall between the 11th and 12th statues on the left of the entry hallway of the Grand Palace”, that's enough knowledge to correctly target your spell at the illusion the wizard put in that exact location and reveal the secret door (or whatever is there). This choice would be “I stand in front of the wall between the two statues, and I target the magical effect that is right in front of me.”
Other than unusual help like that though, you generally need to be able to locate your desired target, which will almost always require some kind of perception on your part. We can't literally require “perception” in all cases though, because being too literal about that though would rule out some cases like the above where you obviously know enough to target the spell correctly — and we don't want that. D&D 5e is, after all, supposed to be sensible rather than literal-but-counter-intuitive.
The examples in the question lack targets… so far
So you could target something you can't see, but in the examples, no targets can be selected yet without changing the situation somehow.
Notably, you can't just choose a general type of magical effect and hope it is somewhere in range, because that's not targeting an individual magical effect. Dispel magic is not an area of effect spell! Just like you can't choose a magic missile at “any orcs in the room”, you can't dispel magic “any invisibility effects in the room”. You have to be able to pick a specific target and cast your spell at that target specifically.
That means that in both your examples, you can't just cast the spell at nothing, say “Invisibility, I choose you!”, and have dispel magic find the invisibility effect and dispel them just because it's within range. You didn't pick your target! Instead, in each situation, you have to do some more work to acquire and choose your target:
You have to locate the invisible hiding evil wizard somehow; most likely by using Perception checks (but divine intervention would work too). Once you positively acquire the knowledge of the position of the evil wizard, then they can be targeted by dispel magic.
You can't can't dispel magic on “any and all invisibility effects in range” because that's not how targeting works — that's not one, as dispel magic requires, that's multiple. You would have to pick one instance of an invisibility effect somewhere specific in the room:
- Try to detect the presence of invisible things in the room somehow,
- Then, if there are any, locate one of them specifically somehow,
- Then target dispel magic at that one magical effect specifically.
Anything else isn't targeting, it's throwing a spell into the wind and hoping it magically does something its description doesn't say it does.
Best Answer
Immediate actions are defined as follows:
Immediate Actions start in the order they are announced. In your example Wizard A starts casting Apocalypse from the Sky, Wizard B cast Celerity and then combust, then Wizard A casts Celerity and counterspell.
This would then resolve as the counterspell countering the combust, and then the Apocalypse coming.
So in answering your numbered questions:
1) Yes
2) Once Wizard B announces his immediate action it has started and would complete except for the counterspell of Wizard A. Wizard A's counterspell starts a tiny bit later than Wizard B's combust. So no issue there.
3) See 2 above
4) Celerity would cause Wizard A to be dazed (unless he was immune as you have suggested). If not immune I would allow him to complete his Apocalypse spell but I think the RAW answer would be that he could do nothing after the action he gained from the Celerity.