T1 enemies do normal damage almost exclusively.

At T2, where Jelly King is, monsters can deal split damage (normal/piercing, etc)

At T3, monsters can do pure elemental, shadow or piercing damage. Or split damage.

You're just wearing armor that's not suited for the kinds of monsters you like to fight.

http://wiki.spiralknights.com/Damage

You need to fight more gremlins and constructs. Pick gremlin or construct strata.

Nothing is mandatory as long as you adjust your strategies to deal with it. Just practice more and you'll figure out how to use nearly any equipment, but there ARE some pieces of equipment that just make life easier in certain areas.

When you go to a gate you can see the 6 stratum themes at the gate console. When you go to pick T1, T2 or T3, you'll be shown the kinds of floors in that tier. The floors BEFORE Basil are all one theme, and the floors AFTER Basil are a different theme. Read the floor descriptions to get an idea of what kinds of things will be in that floor. For example: Deconstruction levels are full of gremlins.

You just have to learn what you like to play in. Every once in a while you should change things up though, and try different play styles with different equipment. It's more fun to try new things.

Vog Cub and Owlite nubs :O Yea i said it. Looks like not only me, no, also the producer are sick of seeing all the people useing the same equip T_T There will come more Shadow and Pierce damage, they said. They wanna nerf Ele so you must craft new things, they sell CE.

In my eyes a pure Ele def ist just laughtable. I use Pierce, Ele and a shadow shield normally. Its the best allround option. I often see Vog Cubs getting hitted once by spikes or jellies or howlizer heads and they are immediately at 50% health :D kinda funny xD

T3 is more about not taking direct hits than beefing up armor. That's why I've been able to ignore piercing for so long, because most of the time it's easy to dodge piercing monster attacks. I agree with Dirt, this makes other sets more likely to be used. That said, now you need a set of armor for each strata or you will take terrible, terrible damage when you get hit. =P

If you exclusively wear a set that has more than two defenses, you're halving the damage reduction you're going to get against either of the typed defenses, as well as your status resistances, and in tier 3 especially you want to maximise your defenses. It's especially bad to gear up against multiple defenses if you're going into a monster family strata - if you know, for example, that most of the monsters are going to be constructs, then splitting your armor into vog and skolver is a bad idea. Keep pure sets.

Having pure sets works ok, but not great, for monster family strata, but status type strata can easily deal out any type of damage, sometimes every type of damage in a single floor. A fire themed statum can have fire greavers/devilites, alpha fire wolvers, fire retrodes, etc. all on the same floor and combinations in the same room.

Even on monster family statra, it can be bad. I made the mistake of using just normal+shadow armor on one of the undead stata with the the candlestick levels. Sure enough, there were also wolvers/jellies on some of those levels, gun puppies on others.

*NOTHING* will turn you into a brick wall in this game. Having something like a divine veil + skolver means that when you mess up and get hit, you are likely to survive to the next hearts, rather than being crippled and having to play super cautions, avoiding options that you could normally use.

OK, I confess that the divine veil + skolver set that I have has UVs to boost the Normal defense quite a bit, and that helps, but I think I'd still find that set to be more functional than the pure sets that I can put together for a large percentage of stata. Sure, the JK run is almost pure normal/piercing, and some of the fiend levels are almost pure normal/shadow, and the FSC is lots of fire with the rest being mostly normal/shadow, but those are exceptions. Granted, people play those exception stata a lot, but I've gotten pretty bored with those.

*shrug*

To each their own.