1) Typings:
Grass as main type. There are grass mons enough to overcome their weaknesses.
Flying as back-up type. Not as present as Grass, but there too to patch thing up.
If necessary, include some "forest defenders" mons. No specific typing, just mons that would flavorwise fit. depends on your needs tho.
In other words: Grass and Flying are good enough for the base of it. But don't limit yourself to only them.
2) Lackeys:
Grass mons and Flying mons can be worked with. Grass/Flying mons can't -_-.
I mean, Lileep is a grass type and works vs ice mons. Same with Ferroseed. For flying...uh...wingull and the like can also work. But I agree with YAS in the sense that Grass/Flying mons, frankly, suck.
For lackeys, tbh, I don't see the need for more than 2 options. Reasoning is: they are lackeys, they will lose anyway. Lackeys aren't there to "stop" anything really. So they being predictable isn't a bad thing (it is indifferent really). And I prefer to have the TLR plannable (is that a word?) on all instances it won't harm the overall experience.
Also PLEASE, no Fake Out/Encore users on the lackeys. It adds almost nothing to the difficulty, but it gives the player a powerful and cheap mon to work with.
3) Guardians:
2 Guardians work. No need to rely on rng not bringing that mon you need to crush the opposition.
I would be against background enemies or spawns, unless the latter is there as a anti-stall insurance. Spawn mons are...eh, I just don't like them very much. We can make ends meet with arena mechanics alone. No need to risk everything by including either a mon that can be used to stall against (most spawn) or a free damage user/support mon that, IMO, makes things harder in the least interesting way possible, as, most of the time, your best plan vs a background enemy (and I speak from a long RE experience with them) is "let's move on before this thing deals too much damage". And that kind of unescapable opposing mechanic lacks finesse and doesn't add much to the overall experience. It is just mean. Again: unless it is there as an anti-stall mechanism.
I don't know which mons would be good for the job. Depends on the arena-theme you have on the TLR. If no theme, then any combination of Grass + Flying work fine and dandy. I would just care to include at least one mega pokemon, as I feel the end result deserves one, at least if the player is going for Skymin.
4) Boss:
I suggest Togekiss + Skymin on an arena with sticky web, a "Serene Grace triples secondary effect chance" effect and a mechanic that turns Air Slash into a spread move.
<_<
I don't know. If we are playing by Skymin's strengths, that is the only mechanic I can think off :/.
As for spawns and background enemies, I am not as strongly against them here, as it is the final encounter. But I feel we can make ends meet through ther means.