>The first fight will be more "narrative" than "tactical"
Sounds like a good spot for a skill challenge. The rules in the DMG are rather ill explained, so use this:
runagame.net/2013/08/4e-skill-challenge-example.html
> I plan on a single big armor as a Brute and 2-3 living swords to act as his support, trying to strike into fleshy backs of the wizard and cleric.
Add some interactable terrain/doodads, like weapon racks/shelves, armor suits, things the characters can use as cover or tip over on to enemies/to create difficult terrain for the animated armor. Maybe make it so the animated armor can pick up weapon racks and throw the weapons at them as a (possibly AoE) ranged attack. Read up on improvised actions, use the table in the rules compendium if you can find it.
>The tunnel is long abandoned and they'll meet a low-tier Myconid and a whole bunch of spores growing from walls. The Myconid himself is a pushover, but the problem with this fight is the spores, which are a bunch of 1-hp Minions that each, however, shoot a low-damage acid that has 2 squares of range - the damage is low, but if they prolong their stay or position improperly, it can accumulate and provoke more attrition.
You may want to stat them as traps instead, but minions work too I guess. I feel like this could possibly be cheesed by ranged weaponry (or just by running away; it doesn't sound like the myconid would be too keen on giving chase), so make sure the tunnel has some twists, turns and stalag-... whatevers to use as cover for the shrooms.
>When they emerge at the other end it will turn out that they were tracked by their oppressor's jungle man - think Kraven from Spider-Man - and his pet snake
I feel like the snake should be a constrictor, wrestling with those is fun for the whole family.
Also, maybe have him set a few log/pit/wire/noose traps around. Maybe have the snake grapple and drag allies into spike pits or hold them still while a log trap swings down.