The E100 takes a bit to get used to coming from the E75. Like I said, it was about 100 games before I began to actually like it. At first, I thought it just felt bad compared to the E75 and was kind of a big disappointment.
With the E100, it has strong armor, but it is really flat. So you need to use angles. Now, because it is flat, if you angle it, that makes it really difficult to penetrate. If you put your armor flat to the enemy, it makes it really easy (particularly for HEAT).
This matters a lot for your turret. Never aim at the enemy except when you are preparing to fire. The flat part of your turret is easy to penetrate if it isn't angled. However, angled it is difficult, and you can angle a lot because the side of your turret is well angled and bouncy. This is different from some turrets where you don't want to expose their weaker sides, even slightly, so you keep looking right at the target. E100, don't do that.
Also, hide your lower plate whenever you can. It is massive, and a big weak spot.
The 150mm gun (which is the one you want) has lower pen than the E75's gun, so you have to get used to that. Plus, you are at a higher tier. If you run into something with a ton of armor, probably wise to switch to HEAT. However, how I use it is to get stuck in at a spot where I can bounce a lot of shots, but then engage targets of opportunity that are hanging in the backfield.
In a recent game (where I got 8k damage and fired zero heat), I tanked shots from an IS7 and IS4, but never shot them. I was firing at the softer mediums and 183 trying to hide behind them, and let those on my team with higher pen deal with the heavies. That alpha wrecks softer targets.
Always be wary of arty. You are a huge target, so try to be some place you have some cover.
You are also tall. This can be an advantage to fire over things (rocks/wrecks/etc.), but also something to keep in mind as you can be shot at easier in return.
That's all I can think of for now.