A necro mage is a hard template to master. If you were ok at necro you will be an amazing mystic as it is an easier set of spells to master with magery. That is in my humble opinion. One thing that is hard to understand is to beat mysticism you have to be mysticism. It's pretty lame in that regard, although a ward removal talisman can counter the worst of it at times.
Mysticism has a few awesome utility and group fighting spells.
Sleep and Mass Sleep are a form of "paralyze" that forces the person hit with them to walk. I also believe it lowers your casting and swing speed. (not 100% on that). Sleep a target and allow your archers/throwers to attempt a dismount. Need a little space from a dexxer/archer? Sleep to buy yourself a heal. Mass Sleep is 100% to work and does a small 2-3 hex area. It is possible to sleep multiple targets at once this way. Sleep itself does a direct mysticism vs resist check and if you mysticism is higher it will go off. The advantage to this spell is how quick it is to cast, but I find that I often use mass sleep to ensure the effect. This spell has a cooldown so you can't spam your target but i believe it is every 5 seconds you can reapply.
Purge magic is one of the best and most under utilized myst spells. It does a myst vs resist check again, and if you are successful you will remove a benefical buff from your target. Vampire form, stone form, wraith form, bless, reactive armor, protection, etc etc etc. Any buff is subject to being removed. It does have a slight cooldown similar to sleep, where you can't 'spam' it.
Enchant is a pretty useless spell, it will add a hit spell to your weapon for 2 minutes. The duration is too short to make it highly effective and 90% of people who are using a weapon have a hit spell already. At 120 myst you can get 60 hit spell but the duration is just so low. You can pick your hit spell, but it's pretty useless overall.
For offensive spells the thing that gives mysticism it's power is it's burst damage which is achieved threw spell plague.
Spell plague is a DoT spell that triggers off other spells. It's a little confusing at first perhaps, but once you cast it the spell will do an initial damage, then the next few spells cast have a chance to trigger another stack of damage from the spell. This leads to devastating combos. Explode Spell plague nether cyclone is a standard combo similar to the explode flamestrike eo pain spike the necros all used. You follow up with the spell trigger bombard you had mentioned liking and you can see the power of this combo at times. It used to be hail storm instead of nether cyclone, but hailstorm has been nerfed a bit and isn't as effective anymore. Another thing to do is cast spell plaug then follow with magic arrow/fireball spam. The spell trigger will trigger off the ma/fbs and you have a good chance of locking the target down making them unable to cast. The part of offensive is hard to explain but if you played necro you would layer strangle then go into some sort of smaller spell combo/spam. Same deal except spell trigger won't trigger by itself (like strangle).
Nether cyclone and hailstorm I will explain now as I mentioned them with spell plauge..
Pretty basic .. "earth quake" AoE spells that you can target. I believe it is 3 tiles again for the AoE but I am not sure. Hailstorm was highly effective until it now splits damage based on the amount of people. If you hailstorm into a group of 10 people it ends up doing 5ish damage. The nice thing is it's a large cold damage spell which magery always lacked. Nether cyclone does chaos damage but hit's the entire group for a large amount. The disadvantage is chaos damage is random and therefor you can do very different damages with the same spell. (mostly pvm)
Nether bolt is a new magic arrow, I prefer old magic arrow but i can cast nether bolt and gain mana so it is useful at times. It is chaos damage again so you never know what element it may choose.
Eagle strike is an energy fireball and is nice to use if someone is running a high fire eater suit. I've been using this spell more and more, it has really grown on me.
Bombard is similar to an e bolt but is physical damage, another major element magery lacks. It has a chance to para the target, a myst vs resist check again, I've never really noticed it going off but I rarely cast it aside from spell triggered.
Mysticism only gets one form, Stone form and it can be very powerful. In this form your resistances go up 5 in max resist (75 caps) and makes you immune to all poison and immune to 75% of necro curses. Super lame and the worst part of mysticism if you ask me, but it allows anyone to be effective on a mage. The downside is you are on foot but this has been used to your advantage if you have bolas. It also takes 2 fc away, but most people go into protection anyway.
Spell trigger and healing stone are great survival spells that should always be ready to go. Spell trigger stores the spell on a stone for an instant cast. The bombard and cleansing winds should really be the only ones you trigger. Healing stone is a renewable healing potion and gives mystics great survivability in large group fights and/or after a death.
Cleansing winds has been nerfed repeatedly yet is still highly effective for healing. It is perhaps the most "noob" friendly heal ever given to a mage. It is a cure, remove curse and heal all in one. This spell in protection and rock form is why you must be myst to beat myst.
Finally mysticism gets 2 summons
Animate Weapon is mostly useful for a meelee mystic, it summons a "blade spirit" that does damage based off the weapon you have. Something like that. It is only used to grief people in houses really as it's hte only summon that can get inside a house now.
Rising Colossus is an amazing summon for the mystic. It takes 5 followers, so humans/elves are on foot but it is similar to a few demons combined. Very strong and considered the standard summon these days.
Compared to necro mysticism is pretty simple to master, it has some powerful spells that get abused, basically a mystic in the group can spam spell plauge and be highly effective. Sleeps and purges are great for helping others and you get the best heal in the game. In choke points, stone form and RCs are powerful, followed by NC etc. Mix in magery for heals fields curses etc and it can be a powerhouse.
I prefer to run spellweaving with my mystics for various reasons that I do not feel like explaining right now. But it gives you a lot of extra utility and power.
my GoR and Shield and pixie plauge is nasty as hell vs a ninja or some other no resist dexxer. But as stated, it depends on your group and your server. I don't have a hard time getting a 6 circle which is key to playing a weaver. I also play with a lot of dismounters so sleep works out better in the end vs eo para as most have a box. It all depends on the playstyle