Tip for HAN-D's Kill Ten Enemies achievements: kill a magma worm with it. Not 100% sure why it worked: maybe I was just lucky and there actually were 10 other enemies there (but I don't think so, it wasn't during a teleporter time and most of the other enemies I saw there were the colossus bosses), or if I happened to hit 10 segments at once with the killing blow, or if I killed it with the forced reassembly and its death counted as 10+ enemies no matter how many segments you actually hit with it. But I was surprised to get it. I considered that it might have worked this way but it was also possible that it wouldn't.
Also, does anyone know if you can just AFK after a teleporter to get his "survive 25 minutes above 75% health" achievement, or does it need to be during a not-after-teleporter time like the enforcer achievement?
From what I remember, you should be able to AFK for the 25 minutes, but I don't think you can dip below 70% health at any time. Basically, clear the first stage without once going below 70% health, find a corner or something, AFK until 25 minutes.
The reason the "kill 10 enemies" works with the magma worm is because the magma worm is actually a bunch (20) of entities all slapped together and given a fluid motion that actually looks petty awesome. So if you hit more than 10 of the entities with the hammer, you should be able to get the Arms Race unlock pretty easily. As for the star looking item, I'd have to look at my list later. I've got most of the items unlocked/grabbed. The only ones I don't have are the two of HAN-D's unlocks, one on the miner (Hard Justice or whatever), and the last Commando unlock (which is called the Hyper Threader and is amazing and I need to touch the third teleporter without being hit aaaaaaaa-).
As for secrets, you should look for a fence on the top left area of the Frozen Valley. Press up a lot.
EDIT: Found the "Star looking thing" mentioned earlier, it's called the Chargefield Generator and it's a Mercenary Unlock.