Mengsk is my new favorite commander. He shines very late at lvl 15 with masteries. The masteries makes him pretty insane, with the quickest fast expand and OP calldowns masteries.
You can adapt to everything, anythime. You can win a game without loosing a single soldier, or turn them all to ashes while you make great use of your royal guards.
Fast expand on all the map you can, with masteries you can pop 2 bunkers full of soldiers first second of the game, it makes you able to clean the rocks, turn your soldiers into workers and start second CC. First, send a SCV to the expension rock so you can drop the bunker near it. This way, you can turn your soldiers into workers right where the rocks are.
If you can’t fast expand, turn thoose soldiers into workers, build raffineries with 3 workers each, build a barrack with 4 workers, then build a factory, then a starport and form a marauder with a medivac. Meantime continue to while make soldiers and bunker. You can defend the first wave easily ; then go clear this expension. Tank with your maraudeur / medivac, and focus dangerous targets with your soldiers. You can clean every extension alone this way (or you can try bunker rush, it might be quicker but it’s way harder to execute on most maps.
On cradle of death, you can pop your bunker just behind first construct then pull ennemis to them with a scv reparing.
On miner evacuation you can pass by the right ramp with a scv. (you might want to make a bunker first in your base to continue scv production, use 4 to 5 scv to build the first bunker quicker).
On malwarfare you can just send 2 scv and drop your bunker in front of the ennemy. On scythe to amon, you might send 4 scv between the 2 lines of ennemy, pass by the top mineral wave and push from the top. It may be the hardest one. Vermillon problem is easy if you pull corretcly + you can defend the first wave from the bunker on your extension.
In all maps, I would go 1/1/2 like they said above, having 2 starports make you able to make enough watchers / medivac.
Once I have my 2 bases full and my 1/1/2 I make 2 watchers, one on each scv lines, later I would add 2 more, with micro, you can always have a watcher over your army, making you able to use every single abilities on your top bar and be at 100 with everything on cooldown.
If you want to go canon instead of making an army, you can still make 4-5 medivacs if he have bio and 1 watcher to help your ally push.
If you plan on doing mass canon, do it only on maps where you can hit near everything from the center. It works quite well ; but you can’t rush maps and if you have a swann or a Karax with you the game might become a litle tricky on offensive maps.
The first upgrade I rush is amplified airwaves in the fusion core ; wirebender is totaly right, then I usually upgrade my medivacs, add a blackhammer and a pride of augustrad with upgrades to my bioball army. Use few weapons at first, 2-3 firemans can tank a lot of damages for you, when one die, a soldier pick it’s weapon, it can save you 50 supply in a hard fight. I add a 3rd CC to make more soldiers until I have 200/200. If you want to add more royal guards, juste kill some of your useless soldiers or make them push.
Mengsk have powerful adaptation, use the most effective royal guards and weapons against ennemy composition.
Never use workers to build bunkers since soldiers can do it pretty well. If you need to repair, you can convert soldiers into workers at a bunker.
With upgrades from the enginering bay, it makes you able to bunker rush everything and defend against everything, everywhere (and you can even call bunkers from space, what are you asking ?) You can then add tanks and medivacs and make you soldiers move in the bunkers where they are needed. Put watchers on every line of bunkers and you can defend everything by your own. You can solo oblivion express with your army split in half on each line and you can do all defensive maps pretty easy.
I never do weapon and armor upgrades on map I hope to finish before 17 min ; but I build 2 enginering bay on long lasting maps and reach 3/3. With the armor upgrades from the blackhammer and medivac support, you will roll on everything with just your bioball, you don’t even need the help of royal guards or calldowns.
For masteries I rececommend : 20/10 (since 30/0 is too much if you micro you watchers correctly ; if you don’t, 30/0 is better) ; 0/30 (except if you use mass canons, you would want to go 30/0) ; and 25/5 (or 30/0 if you want faster first zerg wave on difficult mutations).
For leveling Mengsk, the easiest method is to first build a bunker with 4-5 scv at 15/15 supply then make a 2CC to fast expand with 10 workers at 22 and then turn your workers into soldiers to clean the rocks. If you can’t fast expand, I hope your ally can clear it for you, because you will have it hard. Maraudeur / medivac rush is your easiest option. Don’t even try to clean it before first wave.
Always beware of your medivacs. They tend to stay far behind your army. When a soldier see an ennemy unit, it attack them, but your medivac won’t follow. If you have good micro, you might want to add all your medivacs in the same group or make them on follow mod (it means you never use F2 and it’s very useful with mengsk ultra fast soldiers creation).
I usually don’t make ghost (except 1 against protoss and abilities army composition for emp) because it require too much micro (you allready have to micro your watchers, medivacs and avoid everything that can instantly kill your soldiers) ; but if you are really good then you can mare more ghost factories to have more nuclear missiles. At lvl 3, immolation break every bio attack waves.
Lvl 3 sieges tanks are exceptionnaly good against air in a medivac, but your medivac can’t cloack anymore and it still require a lot of micro if you need to deploy them and put them back in your medivacs (way harder than swaan hercule since you can only have 1 tank for each medivac).
Maraudeurs are wery tanky and can be healed from your medivacs, they do splash damages and slow ennemies. Very useful, very strong, near your only viable option against mecha armies. More importantly, it’s the best way to make use of your medivacs drop, making you able to move quickly and pass over rifts.
Sky fury can tank pretty well at lvl 3 with good dps; if you don’t have swaan, raynor or kaarax with you you will want to have workers always follow them. Take care to macro them well.
I don’t see the utilisty to have more than 1 blackhammer, and it’s useful only on offensive maps where you don’t put your soldiers into bunkers. Pride of augustrad have 3 yamatos that fire in rapid succession and triple tacticals jumps ; making you able to snipe every objective with you calldowns.
Note : the first upgrade on the enginering bay doen’t seem to drop soldiers on rally point (but it drop workers correctly)
Zergwaves units are taken into account when you deploy your watcher. With a 200/200 supply, 100 ennergy zergwave and correct upgrades + masteries, when you deploy a 100 ennergy zergwave under a watcher you can call a nuclear warfare before the end of the fight (well it’s pretty overkill, you might want to drop bunkers instead for the soldiers inside.)