The battle tower trainer will raise the level cap of your skills, but does not improve them directly; you sill need to put more skill points in yourself.
The trainer can also allow you to unlearn all of your skills (the slayer skills that start on level 1 will go back to that level), which you can then re-apply as you wish. The cost for doing so doubles each time, but at least in D2:ED there was plenty of gold available, which could let you reskill up to 4 times, or so (most who took advantage of that option only did so once or twice).
You can only have one summon active at a time, and after you can learn the healing spell (level 15 IIRC) I'm not sure if you would still need the ghost. The demon is stronger than the undead summon.
The Summon Mastery skill increases the power of your summons, but I don't know what form of boost(s) it provides.
