Hiring either Blackguard or Ayden as your main tank will give you a huge early game boost over the lower tier mercs, although with some gear upgrades and training the low tiers can handle most of the same things the top tiers can eventually.
Even with no training at all. With an El Cheapo team (Bud-Loren-Mr Doyle-Aiko-Yourself, or with a slave Quartermistress instead of Loren or Aiko) you can clean the map and keep it clean with ease, except for the group that appears at Preacher´s Pond and 6th+ level bounties. Just buy Combat Armour for every one and Duraplate for Mr Doyle and you will do fine, with very low maintenance and buying your ocasional Healing Packs at 55O price.
In my current run I have started with a 30 ranged combat skill MC. The only training in combat I have made have been some to raise it to 40 (5 points, maybe) to open the Arena, with Rebecca (so irrelevant for those purposes) and some when I get Aria, to raise her to 70 and be able to give her a Coilgun/autorifle without recoil penalty. Bud with a Plasma (once he get 60some skill), Mr Doyle with a Coilgun (that sometimes he don´t fire at all, he closes to use the cane if the combat it´s easy), the girls with rifles and yourself playing jocker, either with a rifle or a coilgun, and you have a combat team that it´s good enough to win, cheap on ammo and low maintenance.
I have used the High Tier mercenaries, of course. I hired them (Ayden and the Black Guard) once I get Aria and one or two plasma guns from 5th level bounties in order to make the Arena Run (you need at least 3 plasmas in order to have a chance to win the last round). 8-9 days of heavy use. In this 8 days I made with them the Arena Run, the Aria Quest and cleaned the Black Scorpions from the Japanese district, and also use the power boost to open the orphanage to invest . After this, I dismiss team and stick with my cheap team. Well, until the dammed Gotzilla appeared in Cassey park. The I boost myself to Duraplate and 150 hits and rehired them for a one day operation.
The trick it´s
1.- Do the cleaning of the tribemen in Preacher Pond by yourself. It´s a 3 days operation (get up, do the daily bounty, go to Preacher pond and fill your team with captured slaves), and you get 6 slaves and 4-5 skill points, spread across your team per day, at the cost of some tendisteam and rifle ammo. Well worth your time.
2.- Once you have opened the arena, go every day to make two easy combats per day (until your team it´s strong enough to make the difficult ones suffering only one or no hits back). Again, don´t see it as a combat, but as a paying training ground.