Grenade launcher! Two of them. Your welcome.
He can use the Haste bot(you can only have the drone and two bots at the same time) the haste bot is one of the best party support in the game.
He can have two +3 to haste bot weapons in his second weapon slot to cast him and then switch back to his main weapon, the haste bot keeps the +6 then if at least 1 point was put into it.
Wich means that you can save 6 points there, when you get a helmet with +1 to all engi skills then even 7 points. And no skills cant go higher then 10, its a bug that it shows a lvl 11 improvement at 10.
You should use aura of renewal, aura of defence and aura of deflection. That with the passiv skill that you can have 3 auras active makes you near unkillable. Combine that with Anchor(If you use it) and your are good.
WARNING: If you go fulltank dont put points into Anchor. But even that is up to debate.
Here a skill tree example(Dont put a point in to aura of power! And just two points in Prayer of healing, so that you get Great Defender! Its just an old outdated skilltree thats why it is there):
https://london2038.com/skill-planner/#Guardian,50,50,0,v1.00001010000000409900163010000001090,440000040400040
Everything not max! because you will get one day a helmet with +1 to guardian skills.
Shield of faith 1 point. If your Shield goes down use it, it will be full again and combine that with shield booster wich you can buy from the vendor, you can put them into 3 of your skill bar for example(they do the same).
You pull everything together, tank it and keep every mob at you, goes into Anchor(If you use it) and then he and you give it hell.
And dont worry if you put points wrong or so in the skilltree, just grind enough money to buy a skillretrainer from other players (500k to 600k i would pay) or farm it yourself in SH (Moloch boss) but thats later.
PS: Dont use Atribute points unless your gear says so. Keep them you need them for your future gear wich you wont now what stats you need for it. But again you can also buy an atribute reskill for even less.